I have a question third gen

how would a modified 2.73 rear end paired up with a modified 700r4,2200 stall convertor do with a efi 408 sbc produces 533 tq motor as far as performance and mpg.i am try to get good high miles and street miles also

 

Weld the axle tubes, elminate wheel hop or prevent it and its all based on luck.

 

how would performance and the gas mpg do with such a highway gear

 

It really depends how big your wheels/tires are. With nothing huge, you might want to go with 3.08 gears. That way you'd get better MPG's around town and more bottom end kick with no noticeable loss of highway MPG's.

 

yea i was kind of leaning in that direction,but with over 500 ft pounds of torq you think there will still be lag.the wheels that i am running are the stock trans am wheels the 15 inc rim etc

 

who out here with experience can answer my question

 

It's been a year and you still don't have an answer?

If you have a lock up converter then you mioght pull this off. If you have a non lock up converter then you are probably going to build a lot of heat in the transmission with the 2.73 gears and 2200 stall converter. You also need to consider the cam and other parts of the engine. If this engine is not built for low RPM torque then you are not going to be happy with this.

 

its a 408 sbc tpi motor it has more than enough to push just about any gear but i was really wonder about the gas mileage with the 2.73 gears

 

Pretty much the same as with 3.23, 3.42, or 3.73; won't make any material difference; could even end up WORSE with the stupid gears, since it'll force any motor that can produce 1.25 HP per cu in into a range of RPM where it will be TERRIBLY inefficient.

Putting a nice warm-fuzzy buzzword like "modified" in front of every part description won't change the arithmetic.

Of course though, with TPI on top of it, doesn't matter what the motor is "rated" at, or even, what it did with some other intake tract; it'll be lucky to make meaningful power about 4500 RPM same as any other motor w TPI on it, and lucky to clear 320 HP because it won't be able to spin up. Since after all, HP is a function of RPM, and torque at that RPM; if either one is limited, the product will be similarly limited. Hard to make the same HP at 4000 - 4500 that it might have been able to make at 6000 - 6500 if it had an intake system that makes sense instead of TPI.

Fork in the road here: you want gas mileage and/or TPI, that's not the motor for you; you want that motor, gas mileage and/or TPI is not for you.

One or the other. Take your pick.

 

I am not asking how would the 2.73 would compare in mpg vs a 3.73 or 3.42 or 3.23 thats not the question.To say that it would be same as a 3.73 or .3.42 or 3.23.well it has to be a difference some where.each gear ratio changes the behavior of the engine that was mention above.

we are not talking about a 350,355 ,or a 383 or hell even a 400.everybody knows that a 400 block with a stock tpi intake on top will throw anyone in the back seat because of the extreme amount of torq that the combination makes.we are talking about a heavy modified roller 408sbc controlled by an ls1 pcm, and lq9 coil packs from efi connections with 210 runner head cnc aluminum heads.so its very efficient for its kind.the motor produced 497 ft of torque and 399 hp on the mustang dyno chassis in texas.this was with a ls1 stock tune,i still have to buy a wide band o2 sensor for it. not to mention its not fully dialed in yet. the lower intake has been sent to recieve the mega ported and siameased 3 inches in deep in the intake treatment.my slp runners are having the dividers removed that are inside the runners and mega ported aswell.

It will make even more power once those issue are fixed.the intend of the motor was not intend on being a high rever screamer if thats the case i would have choosen a 4.8 vortec and have it buit to rev at 7k.my 408 is torq beast and it does make power past 5600 rpm with the efi connection efi setup on it. so those gears are the least of my worries I have enough torque to run anything gear that i want.

What i am asking is what type of mpg would it make with my combo with 2.73 gears posi gears.From some guys that has first hand experience with such gears with high torq motors.Opinnions are welcomed but not required.

 

The real question here is, why are you focused on mpg with that high performance engine anyway? Mpg goes out the window when you build a big, powerful engine. The bottom line is, it's going to drink gas like it's dying of thirst regardless of the gear you have out back. That big of a motor, you're pulling in more air. More air = more fuel. If you want mpg, get another car. Just my

 

Not sure how this can be put ANY PLAINER:

As to what that ABSOLUTE NUMBER might be, will depend WILDLY on your tune, on driving conditions (city vs hwy etc.) and EVEN MORE WILDLY on how the driver drives it. I think we can confidently "guess" though, it will probably be between 6 and 20 mpg; most likely in the 12-14 range.

If you want gas mileage, that's not the motor for you.

If you want FUN, you're not going to get gas mileage.

Those stupid gears aren't going to make a material difference to the combo's gas mileage. All they will do is reduce the FUN you'll get out of the motor. It's beyond a lack of common sense to spend that amount of money and effort on a motor, and then cripple it with that gearing.

This is a combination of several bad decisions and then asking a question that doesn't fit the situation at hand, regardless of whether or not "that's the question you asked". This might have something to do with why you haven't got an answer that you like in a whole year. Instead, now that you've had a year to finish building it and tuning it and you even have it in your car and running (chassis dyno), instead of keeping all of us inferiors out here in suspense, how about YOU tell US what gas mileage it REALLY gets.

 

Roller cam tells me it's got a lot of duration and ment for higher RPM use. This is NOT going to work well with 2.73 gears and it's really not going to work with 2.73 gears and overdrive. I don't care what the peak torque is, if it isn't making that peak torque at a very low RPM then it's going to suck with those gears and overdrive. Fuel mileage is going to suffer because of it. You want to gear the car to run in the RPM range where the engine is most efficent at highway speeds. The 2.73 gears might get good fuel mileage with overdrive in a lighter car with a stock engine that makes peak torque at a low RPM, but a 3.73 might get better fuel mileage with an engine that makes peak torque at a higher RPM.

 

An engine and transmission gearset together are going to perform "best" overall in a certain range rpm which is determined by the rear gearset. You could run a 2.41 rear but you wouldn't gain enough to make the downsides worth it. You could run a 3.73 or higher, but you ****ed your combination's overall power and torque by choking it with the long-runner intake (even with porting.)

If the vehicle is driven in a narrow range of speed as a highway vehicle, 2.73 is around the ideal gear, but slight gains could be had for mpg by going to a 2.56 or 2.41.

If the vehicle is going to be driven in a situation where it will stop and go or regularly vary speed up and down, a shorter (3.42) gear allows the combination reach top gear in the transmission sooner with less effort, which helps mileage.

Don't whip it out for us and remind us how much it's unique and different than anything ever made, conceived, or imagined. It's a mis-matched combination of parts, just like most engines that people who aren't a Smokey Yunick or John Lingenfelter build.

You're going to get low to mid 20s for mpg on the highway if it has good velocity in the heads and is cammed ideally for mileage. If not, you'll get high teens on the highway.

Thanks for the overwhelming details of your build. Pro-tip: If you claim your build to be so unique, you're the one who should be answering the question of it's results.

 

again opinions are welcomed but not required,your two cents are not answer the question phoenix.I didnt ask for " your reasoning as far as to wonder why i am asking my question .nor did I ask about your theory if you dont have any first had knowledge about the gears that i am asking about then save your pennies phoenix.

Hey sofakingdom you were doing good at first answer the question but again,I didnt ask for the rest of your emotions to go along with the answer.If i wanted an internet mechanic to judge my engine combo i would make sure that i ask you.If i want advice from a comedian or a smart @$$ then perhasps i can find you in the yellow page.If i wanted a scientist to answer my question I would have emailed discovery channel.keep your emotions off of the topic of this tread your reply is absent in answering my question.if you never worked with these gears then go find a toliet to flush

Don't whip it out for us and remind us how much it's unique and different than anything ever made, conceived, or imagined. It's a mis-matched combination of parts, just like most engines that people who aren't a Smokey Yunick or John Lingenfelter build.

what you think you are the law or something,(Don't whip it out for us and remind us how much it's unique and different than anything ever made, conceived, or imagined)you sound like a mad step child here,lol keep you dress on.

(It's a mis-matched combination of parts, just like most engines that people who aren't a Smokey Yunick or John Lingenfelter build.)

It may not be to your likings.but i bet pinks slips it would own your setup,jealousy is a female trait jmd,

I only informed you guys of what yall didnt know of the engine.so you guys would have a idea of what parts are in the engine.Now the childish trolling is not what i am asking as a opinion from you guys stop acting like girls ok.if you dont know then say I DONT KNOW.Its simple put your pride to the side


Guys look i didnt reopen my post to have a war started here,some of you are not playing nice or fair .If some of you would read you would see in my early post.That i clearly stated that i am not in country,I am currently deployed in afghanistan I will be home soon to see the end product but yes for about a year i have been off and on back and fourth out here in the middle east.So since i am not there to find out for my self hell i thought why not ask third gen the question.But the stupid trolling comments save them for your wedding date.please by pass if you dont want to answer the question.we are getting off topic here.

 

No, that's prolly true, we're not; we're telling you THE TRUTH. The truth is rarely "nice" OR "fair". It just kinda is what it is. I don't know what you are expecting that WOULD be any of those things; but around here, we don't lie, and we aren't any of us in some kind of awe of a 400. I've had too many of em in my day to think it's something special. We're not here to just worship whatever half-baked idea whoever comes up with just because parts of it here and there sound spectacular. All your big talk and insults won't make THE PARTS work any differently.

You see, regardless of all that other stuff, THE PARTS will not listen to your "will" or your "good idea" if it doesn't agree with REALITY. None of us here are interested in sugar-coating REALITY; we just tell it like it is. Which in this case is, you've built yourself a high-RPM, high-power motor; strangled it with TPI; and now are planning on crippling it even worse with that pitiful gear and totally inappropriate converter. You've let pretty dyno #s FOR SOME OTHER MOTOR BESIDES WHAT YOU"RE BUILDING fool you. One thing I guess most of us here have learned THE HARD WAY, and which we're now trying to pass along to people who will LISTEN to the VOICE OF EXPERIENCE, is that parts mismatches like yours don't somehow "compensate" for each other and "average out" and give you the "best" features of each. Instead, what REALLY happens is, THE WORST characteristics of each part, whatever that might be, become dominant. So what you'll have is a motor that has too much cam for the "tuned" effect of the TPI runners to work, and too much head for the intake tract, which tend to want to make it run at high RPMs; it will be strangled by an intake tract which is barely adequate for a 305 and is specifically designed by the "tuned" runner length to produce power at a specific RPM, regardless of all else; which will then be forced to run 99.9% of the time at about what its idle speed should be by the converter and gear. What you'll get is a plug-fouling, gas-sucking, disappointment, that the only time you'll be able to even tell there's a potentially hot motor in there somewhere, is if you drive down the expressway in 1st gear. We'd like to help you avoid that because as sure as day follows night, that's what it's going to be. It's a mismatch.

Oh - and thanks for your service, I didn't notice that; so I'll cut you some slack on not having it finished yet.

Meanwhile I urge you to fix the mismatch problems. GET RID OF those long runners, maybe use a MiniRam or one of the single-plane systems that are out there; and put a commonsensical gear and converter in it that match the motor's mechanical setup.

And if you don't do want to do that, well then, I don't have anything further to offer. Feel free to hose yourself any way that pleases you.

{edit} And by the way, I do have experience with that gear ratio... the most recent time, it sucked so bad behind a 454 I threw it in the trash and put something decent in the car it was in (3.42 in that particular case)

 

First off, I do have experience with 2.73s. They came stock in my car, and I've raced with them this past racing season. You want my experience? They suck. They take forever to get the car moving. As for mpg, I couldn't tell you. I didn't drive it for gas mileage. There was not one single tank of gas I put in that car that I didn't go WOT. That's what the car is for, just like your motor: going WOT. WOT and mpg don't mix. I don't know how that can be put any more clear. As said, you WILL NOT be happy with them. We're trying to save you a headache.

Second, it's not necessarily "theory." The bigger motor, the more fuel you'll use (unless you run it insanely lean ). It's not that complicated.

Third, I don't know why you're giving us the attitude, when we've given you both our experiences with 2.73s, AND actual mpg number estimates. We answered your question, we're just wondering why you would kill the fun of that motor with a crappy gear. If you want to make that gear work with that motor, go turbo. That's the gear Orr uses, to much success. Don't get mad at us for telling you the truth, and not sugar coating it.

 

phoenix

Its a different between participating in the topic if you could answer the topic question vs bashing they are two different things.I would not off on the far end to say that i am mad because of what you stated.I am just addressing useless information that i didnt ask you for.when you ask in so many ways why would i want to use the gear that i use next ,you asking a question with a question when you really dont know the answer to the question.you have no right to claim that its the truth,when you dont have a big motor,remember i never planned on drag racing this will be one of my daily drivers.needless to say you have no first hand experience with the mpg that these gears put out.I am not trying to drag race you said your self you dont know what type of mpg it can produce and you have a 350 or a 305 not a roller 408.so why are you even making statements if you honestly dont know.if you dont know as you stated what mpg can be made the dont comment because opinion on my combo is not what this topics about.I am reacting to a bashing phoenix

 

I wasn't aware that I was bashing lol. But now that you accuse me of such, I may aswell play the part. There's a few things wrong with this picture. 1. You're using a 408 cid V8 as a daily driver? What even is logic? 2. You're looking for fuel mileage out of a V8, and a big one at that. The two don't go together, what's so hard to understand about that? 3. You're using a horribly mismatched combination that will just give you a headache, and you're so stubborn about it that you won't listen to any of us.

I will say it again, one final time. If you want fuel mileage, get another car. Otherwise, you're just being ridiculous.

 

what is your set what do you have a v6 350 0r 305 ?sense you know everything.sense my setup is so mismatched what is your setup.

 

if you were to take your head out of your a#$ you may learn something.yes you have stated something that everyone knows as far as me choosing a 408 as my motor.

2. You're looking for fuel mileage out of a V8, and a big one at that.

a v8 can get good gas mileage even the big ones its all in your combunation. I have a custom cam that was made to work with my 2.73 gears.yes of course a 3.73 or 4.11 would help my car accelerate quicker Im sure.But i never said that i was looking for fuel mileage out of my v8.You must hear things or little people in your head.

You're using a horribly mismatched combination that will just give you a headache, and you're so stubborn about it that you won't listen to any of us.

you see, when you assuming that you know,everything you make an *** out of your self every time.you dont know what spec cam that i am using what type of lifters that i am using,or what efi system that i am running with setup nor do you know what spec my transmission is.you dont even know what car its going in.

I will say it again, one final time. If you want fuel mileage, get another car. Otherwise, you're just being ridiculous

I never said that i was looking for fuel mpg soley from my motor you keep repeating things that you hear in your head ,you put those thoughts in your head.So I now have to say that your being ignorant and delusional because you seem to have a hard time focusing on the topic.I never created this post stating that i was looking to get hybrid mpg out of my setup.if so that would be wonderful

I wasn't aware that I was bashing lol. But now that you accuse me of such, I may aswell play the part. There's a few things wrong with this picture.

It comes with the territory when your ignorant.The only thing that is wrong with this picture is,your still talking when you cant understand the topic of this thread or stay focus at least.Go some where and drink some lotion you will feel better.

 

That's funny, the first post asks about mpg. I'm going to hold my tongue before I get in trouble for the words I really want to say. I'll just let you go on your ignorant way, and be disappointed with the waste of time and money you'll spend.

My setup is a 350 siamesed TPI, Comp XE268 grind, 3000 stall, TH350, 3.73 rear gear. The setup as a whole is paired correctly to shift about 6000.

Hope you have deep pockets, you're going to need them once your mismatched pile doesn't perform like you want it to

 

Sofaking and others make quite a bit of "sense" but "since" nobody seems to understand your logic on running a 2.73 behind a modified 408sbc that's already unfriendly on gas mileage OR why really consider options to try and acheive better mileage...why not stop posting and go do your own trial and error with what your questioning?? You should have current gear mpg data of xx, so move on to the 2.73 gear as your wanting, note the mpg and compare. You most likely won't be happy with the results. We are all basically saying its a waste of time. Stop taking offense to the "sense" that we are questioning. Good luck!

 

+1

 

I agree with this 100%. If you know it all then why ask? I will say this 2.73's are not good for any kind of perfromance unless you are going to top speed only. I had a 90 Stang GT that I went from 2.73's to 4.10's and it was the best mod I ever did to that car-dropped over a second in the 1/4 and I would say the only reason MPG went down was becasue I could not keep my foot out of it. I will never understand for the life of me why people care so much about MPG. My DD only gets about 17 MPG-20 if I drive like a grandma and I drive at least 60 miles a day.

I was told don't fear the gear and from what I have learned by having steep gears in many vehicles-that's a very true statment.

 

Seeing that your really pumping out Hp with a N/A V8 Mpg's are going to be hard to get. I personally have a V6 though so I cant add much although I was running mid 13's last year picking up 25mpg on the highway to and from the track on 3.23 rear ends gears with the 700r. Its a whole different set up though.

But if you tune it really well, have decent low ratio gears, you can still be pretty fast and get mabey...mabey 18-20 Mpg highway. Whats in there now as for gearing??? Ever think of 3.23's??? There not too bad for preformace for a streetable MPG minded car. I say that youll still be fast because 500Ft Tq like your saying the car has, is damm nice. Tuning here for closed loop, lean cruise, would really help the highway Mpgs regardless of the ratio in the rear end.

But Its hard reading though all the bickering so im not positive you have EFI or not. If its carbed, then MPG's are out the window IMO

 

Watch out Mark, he'll bite your head off for not having experience with 2.73s

 

I got a buddy with a similar build in his 95 Formula (383 LT1, LT4 Hot Cam, AFR 195 Eliminators, built 4L60E w/2600 stall), and he's dead-set on running 2.73's also. He's running 3.23's at the moment. I emailed him a link to this thread and I'm hoping he pays close attention to what everyone has been trying to tell this guy. 3.08's might be a feasible compromise, but I think 2.73's will be a mistake in his case too. I'm hoping to save another guy from a potentially bad decision!

 

There is no way the LT4 HOT Cam will run ideally with 2.73 rear gears even with that 2600.

Over-doing one aspect of a build (instead of matching everything together as a package) in hopes it compensates for any shortcomings of mileage, power, etc. pretty well guarantees it's going to be mismatched and suffer in 1 or more categories. An LT1 4L60 2.73 F-body could be built to pull some great mileage, but added cubes, overcamming for that ratio, and going to larger intake ports means it's mis-matched to that goal.

Ah well, at least they'll all have fun.

 

Mpg went down because your 4.10 gears turn more rpms than your 2.73 gears did.

 

Some interesting responses in this thread....

To the OP if you are still following this please post the build specs of your motor and what trans. Cam specs etc and if you can dyno graph of the torque curve and peak hp rpm. Sorry if i missed it if already posted.

I think the combo will do surprisingly well with 2.73's if powerband is broad and short rpm range like most tpi engines have. If 4 spd auto od then definately will do well.

2.73's are good gears for certain applications. A very torquey but broad power curve motor with properly matched stall will do good. My bolt on ls1 car only picked up .06 in et and 1 mph goin from 2.73 to 3.73 with 3200 stall. Lost 5-6 mpg highway. Totally not worth it. Rpm at cruise is very important for mileage, sometimes even more so than air fuel and timing.

I have done 2 different engine combos using 2.73's. One 401" with small heads 195cc and a 233 deg hyd roller cam. Narrow rpm range, basically 5500-5700 shifts. Huge torque curve. Drove great even with th400 and 4000+ stalls. Never measured mileage but estimated 15 mpg. Never tuned for mileage tho.
Second was same 401" block with huge 245cc heads, big 246 deg cam. All top end 5500+ rpm power. Guess what? Still drove docile. Lots of idle lope, lots of overlap, 2.73 gear didnt care. 3500-3600 stall now based on reduced torque in the low mid range. Everyone would claim it wouldnt drive right. Granted combo performed better with 3.42's but converter was not optimal... If it was, combo would have ran harder but doesnt change way it drives on part throttle and low rpms. Puts around at 1200-1500 all day long at low speeds. Tune helps.

And i have had a 383 with 3.42's get closer to 20 mpg with 700r4 locked converter. 230/245 109 lsa hyd roller. Much bigger than lt4 hotcam and no issues creeping 1200 rpm locked up hill at 20-30 mph. No issues at 55-65 mph neither.

Finally another efi 406 my friend had with a t56, but 3.73 gears / 28" tire. Would cruise 5th/6th gear no problem and got 20 mpg on 5 hr trip to track where it ran 10.4's all motor. 208cc head 250-ish deg solid roller. 6th gr would be like a 700r4 od

 

RPMs have ZERO to do with it. It's about how efficient your motor is at turning chemical energy in gasoline into kinetic energy at a given RPM. It's entirely possible and I would argue highly likely that lugging a car along in overdrive at low RPM's is going to get FAR worse mileage than having the car in its ideal cruising PRM range. Our stock engines are designed to start producing power right off idle, which is why they make such pathetic horsepower numbers. 2.73's are fine for this. But you get any aftermarket cam worth the trouble to switch out for the OEM one, and you're going to get AWFUL mileage lugging the thing around at low RPMs because the valve timing just isnt efficient at that level. You will waste fuel because it doesnt burn efficiently at that RPM whereas it may burn more efficiently at a higher RPM.

Fuel is chemical energy. Gas mileage is just your engine's efficiency at turning it into kinetic energy. It's possible that a 350 peak hp 383 and a 170 peak hp 305 are going to get exactly the same mileage if they're producing the same amount of power in a given set of conditions. It's when you have the RPMs in a range that a raunchier motor isnt happy with or when you have the throttle pressed in farther where the 383 goes beyond the 305's max power output that the real gas mileage changes. There's tons of variables and slight differences and parasitic loss factors here, so you cant make blanket statements. But it's just about efficiency in different operating conditions. Look into http://en.wikipedia.org/wiki/Brake_specific_fuel_consumption if you want to learn more. It has nothing to do with RPMs. If it did then all of our cars would be running with 1.00 axle gears to meet CAFE regulations, but even stock engines would make terrible mileage idling around at high speed cruise.

 

Dont over complicate this. Anytime your engine turns faster you burn more gas. Lower gear ratios (numerically higher) do this. Let's say same transmission,same car but with a numerically higher gear it will make it turn at a higher rpm and burn more gas. Plain and simple.

 

If the motor is 20% less efficient at the lower RPM but turns 10% slower, you end up using 10% MORE gas.

The notion that gas mileage "always" "must" go down as gear goes up, is a fallacy. WAY too many people have found out otherwise. It is PARTICULARLY fallacious when applied improperly to engines with too much cam and too much compression to run efficiently at some arbitrarily low RPM; in reality, the stooopid gear choice results in a pinging, smoking, gas-sucking, overheating, not-enough-timing-advance, SLUG with no leave.

Gear choice should match the engine's optimum efficiency RPM, and should allow the engine to comfortably stay at or near its most efficient RPM as much of the time as possible.

 

But that's not a safe assumption. How can you logically justify that?

We can use this calculator here:

http://ecomodder.com/forum/tool-aero...resistance.php

Fudging with numbers in reasonable ranges gives us a HIGH estimate of around 20hp needed to propel a 3500 lb car at 65-70mph. It may be 40, it may be 15. Depends on a lot of different factors. We will say 20 because now know that is a reasonable baseline estimate.

If you have a manual transmission car you can actually play with this. We can make 20hp in 4th gear, 5th gear, or 6th gear. Each gear will result in a different RPM, but as long as we are maintaining cruising speed, we know we are maintaining the same amount of power output. There are likely small differences in parasitic loss with the different gear ratios in our transmission, but they are small enough they can be effectively ignored. So if we are given the same cruising speed and the same power output, that means the ONLY thing that changes is our RPM's. Which gear will get the best gas mileage?

It depends on where your engine makes 20hp most efficiently. For my mildly cammed engine, 6th gear got WORSE gas mileage than 5th at 65mph when I had the 3.27 gears. I could even verify this in action because I could see my air fuel ratio gauge go richer and I could feel the engine running noticably rougher. Due to the .50 overdrive gear I had, the engine was dropping below the ideal RPM range for my cam AND my intake manifold. There was a lot of wasted fuel, and at these low RPMs the valve events happen so far apart that there's enough time for the cam overlap to waste fuel and send it straight into the exhaust. I would have to go 70 mph just to get it to 1500 RPMs in that gear. I have a fairly mild cam with a roughly 1300-5500 range. My intake operates best in the 1500-6000 range.

But assuming you have a good running engine and a well tuned fuel injection system, you shouldn't see a large difference in gas mileage between 4th, 5th, and 6th gear if you keep your cruising speed the same. There will be a difference, obviously, but it wont be worse in lower gears because the RPM's are higher, it will be worse because your engine makes the requisite 20hp more efficiently at a lower RPM. Modifications to enhance power almost unanimously work to shift the engines ideal operating range into higher RPM's. When you get into performance smallblocks, it's VERY possible with an overdrive transmission that you can get WORSE gas mileage with a numerically lower gear ratio in the axle.

The fun part is that all combinations are different, and there's no hard rule to determine what RPM's an engine will make a given amount of power most efficiently.

Fuel is energy. Fuel mileage is more closely related to energy output than it is to RPM's. You can go down a hill on the interstate and put a car into 4th gear and turn it at 3000 RPMs. Does that mean it's using more gas? No. It's using almost NO gas going downhill because it's not producing power. Power output and fuel usage are directly related. RPM's and fuel mileage are, at best, indirectly related. The notion that the amount of rotations an engine makes has a direct effect on fuel mileage is the result of the false assumption that an engine will always use a given amount of fuel for each revolution. This is obviously false because you can force the RPMs higher in an engine braking situation and use almost zero fuel. In fact you get better gas mileage leaving a car in gear while going downhill than you would if you were to put it in neutral for this very reason. If you put it in neutral, it would idle, the engine would have to expend power to keep itself running. But if you leave it in gear, gravity keeps the engine running so even less fuel is needed to keep it running.

Consider towing a trailer. Due to the extra load on the engine, power output increases despite RPMs remaining the same at a given speed. Gas mileage responds to power output, not PRMs. Gas mileage is depending on engine efficiency at a given power level, and while RPMs affect gas mileage, there isnt a direct correlation that higher RPMs = less mileage. There's a range where efficiency is higher, and outside that range it is lower.

 

idk, i thought my mileage would stay the same or get even better when i swapped from a 2.73 to a 3.42 in my 91' L98 tpi car. Mileage went down a good bit..

 

That's because a L98 TPI car isn't a "performance" motor by anybody's stretch of the imagination. It is plenty efficient in the RPM range that a stock converter and those pitiful gears pin it into. And, its efficiency gets MUCH worse, REAL FAST, as its RPMs go up. That's not what it was built for. It's a tractor motor.

What works for that, doesn't necessarily work for a high-HP, high-RPM, high-compression, high-performance engine with a big cam.

As always, you can get into more trouble by over-generalizing, than almost any other way.

 

Consider self egr action of cam overlap. Egr is important for mileage. You dont want to burn more fuel. Every ve map i ever seen increases fuel with rpm to maintain same air fuel ratio. Its only logical as long as cylinder is filling more with increasing rpm, and increasing load, and based on the combos i have tuned, this is true. Egr recycling of exhaust helps extract the most out of the fuel charge, reducing new fuel added to the mix. This should reduce bsfc.

If you got worse mileage in 6th because air fuel went richer, then theres one issue. Its not a fair comparison. Granted it may not want the same air fuel ratio there but until you try the same you cant be for certain. But its true sometimes the lower rpm is not always the sweet spot. It may lug too much and require too much throttle position and more fuel/less egr effect. Its hard to say one way or another, imo, that rpm has zero to do with it. It some cases it certainly has influence.

I was following a few threads on mpg and engine fuel efficiency alittle while back and it is interesting to see what actually has worked in the past. I will see if i can find them and link them. I think its an interesting discussion and often overlooked in performance hotrodding since hp usually doesnt mean mileage, but i dont see why it cant be

 

The scenario I mention was a 60-65 mph cruise in 6th gear with 3.27 gears. It puts my setup into the 1300-1400 range. That's awful low.

 

While I didn't really compare the mileage difference, I just recently changed from 3.23 to 3.42 on mine because when it's in OD just cruising along at lower speeds I could tell the motor just felt like it was lugging too much, and it didn't feel like it was running optimally. After the gear change, it feels more efficient at cruise speeds in OD. It only bumped it up a couple hundred RPM's, but it still feels better now. Take off is fantastic with either ratio.

 

http://www.speedtalk.com/forum/viewt...hp?f=1&t=35911

http://www.speedtalk.com/forum/viewt...p?f=20&t=31607

http://www.speedtalk.com/forum/viewt...ileage&start=0

Some discussions on tuning for mpg, how to setup build for mpg and such. Some interesting thoughts. Goin lean til suffer misfire and then backing off slightly seems to work well but other factors involved