Well i dont know about yall but here in Georgia spring is almost here (its like 60* outside today!) and its time for the spring tuneup on the 'roc!

Im keeping gas mileage in mind here, as i have a long overdue trip to Panama City coming up and i want to be easy on my wallet on the way down. When I bought the car from Whytspeedevil in Sept i drove it from NJ to GA on like 2 tanks of gas! (this is a modded TPI were talking about ) When i got it here i tuned it for power and although she pulls like a raped ape i lost a lot of gas mileage. So im thinking Ill turn down the fuel press to stock (what is it like 48psi?) and retard the timing back down to 6* (right now its at 9-10*)

Will this make any difference in gas mileage?

Also, i had to install an aftrmkt temp gauge in place of my old one (never could find that damn broken wire!) and i took out the sensor in the drivers side head btw #1 and #3 to install it. That sensor is just for the gauge right? the ECM runs its temp sensor off the intake right? I just want to make sure because if my car is running in closed loop all the time that will KILL mileage!

Anyway, thats my idea. Lets hear some other ideas and tricks for gas mileage....

2.73 rearend gears

i get 9 MPG if i'm very nice and gently tap the gas LOL

I get like 11 mpg in my Z. Thats in traffic.... Strangely, when i get on an open road it goes down....wierd....

I get ~20 50/50 city and highway. Could be better. Easiest way to check all your sensors is to hook a laptop up to it with an aldl cable. Best 60 bucks I ever spent.

Actually you want your car to run in closed loop mode. This is where the engine reads all the sensors to decide the air/fuel curve. In open loop mode, your engine runs without regard to any sensors until the water temp sensor tells the computer when to go to closed loop.

Anyway, I have a Monte SS that I just did the TPI swap to and I am getting about 18 on the highway with 3.73 gears and a 700R4. I think I am toting a little more weight than an F-body but I am not sure. I would say it appears you are heading in the right direction by adjusting fuel pressure. I was thinking though that advancing the timing would allow for better fuel burn off and increasing mileage. Not sure on that one though.

Hope to see you in Panama City sometime. I have a beach house there and go down as often as I can. I also try to get up to the Atlanta - Marietta area too. I used to live there and went to Pope High School from 1987 to 1989.

i was gettin 20mpg (imperial gallons). but that was with a basically stock engine. and i bypassed the TB coolant lines, and now im down to 14mpg. when i had a 350 in there, i had about 8mpg.

With proper tuning of the eprom and invoking Highway Mode, I was able to routinely obtain 30 USmpg (when my L98 was basically stock). And this was driving in the mountains and passing cars. On one particular trip I drove 385 miles/615Kms and used 12.7 US gallons (48.0 Litres).

thank you for the conversion fellow canadian, i see miles and gallons and im like "big whoop?" but i see over 600KM's and im like WOW.

Getting good gas mileage and good power is a matter of matching components and getting your RPM in the optimum range for your cam, it's cruising RPM which is where it gets the best gas mileage.

I get 20 - 21 mpg on the highway and about 12 in city. HWY is at 70 to 85 mph, the city is with a rather heavy foot and a lot of fishtailing, so that's the price for having fun. You need to find out from your cam manufacturer what the cams cruising rpm range is and then match it on the road if possible. If you lower the fuel pressure it might help if your chip isn't optimized for the car, the stock fuel pressure should be 38 to 42 if I remember correctly, I run 49 and it's been so long I don't remember for sure the stock number.

The best mileage I ever got out of a single tank of gas was just over 700 Kms (715 kms to be exact or 445 miles). It was on a trip from Abbotsford to Prince George. Again, this was when my engine was still basically stock and I was trying to make changes to maximize gas mileage all through the tuning of my eprom.

And I passed plenty of cars along the way, so I wasn't exactly "babying" it either. I typically drive 10 km/h over the speed limit, so I was doing 110 km/h (70 mph) most of the way.

43.5 PSI I think.

I got 24 highway at 72mph with 305 5spd and 3.08 rearend gears.

Ouch

10.5:1 305 TBI, ported 187s, Stock 350 929 cam, tuning with lean cruise in the 7747, 700r4 w/ 2,000 rpm converter, 3.08 gears, and P255/70/R15s. I got 23 mpg over 725 miles on a highway trip in the Van. That was on a 105* day with the A/C units blasting on high, and 5 other people with me. We also had the back full of bags, etc. Most of it was running 70-75mph with the cruise set. I was still running heated air off the exhaust manifold, the stock GM TBI intake manifold bored to 2" with a 2" bore 454 TBI.

I've always wondered if changing fuel pressure made a difference to gas mileage. Some say they get better mpg with higher pressure. Some say lower it to stock for best mileage.

30 mpg with these cars and an automatic?

I put $143 of gas into the tank during one day of driving a couple summers back. Had to fill up the tank three times! That was averaging 20 mpg. The surprising thing was that no matter what I did, the mileage wouldn't go down. I had the car at 85 mph for around an hour and at 100 mph for 15-20 minutes. That tank was the lowest at 19.8 mpg.

Converter locked up and overdrive = great mileage.
From 40-65 mph, the car wants to shift back and forth or lock/unlock the converter. Mileage isn't all that good at that speed. I tried measuring mpg at constant 60 mph vs 75 mph and found that 75 worked better. Besides that, the car just feels better at that speed (2300 RPM).
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2.77's....3.73's on the way.

That's gonna hurt mileage. 3.73's are like driving around in 3rd gear with the 2.77's.

Yes, that is one of the "economy" tricks I used - early "Lock Prevention" of the convertor. Also, I used HIGH fuel pressure and then completely re-tuned my VE tables to obtain 128/128 through out the table with the high fuel pressure.

Once I had the VE Tables properly tuned, I engaged Highway Mode in the eprom to leand out the engine to 17.4:1 and increased my spark advance to 47*. A funcitoning EGR is a must if you want to do that. Lastly, I tried to maintain Highway Mode until I hit around 74 Kpa where I then quickly dropped out of Highway Mode (and went back to 14.7:1 and reduced my spark to 42*)...you have to be really careful as you switch from Highway Mode to "normal" and then "P/E" once you go over 85 Kpa.

But you MUST have perfectly tuned VE Tables before you engage Highway Mode and lean out your engine. The savings in gasoline alone paid for my eprom burning equipment.

However, I no longer run TPI nor an EGR, so none of this no longer works.

I think your 666 ci Death Mobile runs on dead souls now.

Looks like you did your research into how to pull off the highway mode stuff safely. 17.4 AFR.....that's halfway between gas and diesel ratios. I bet with advances in technology that by 2012, new F-bodies will be able to get better mileage than diesel cars.

I tried as lean as 18.2:1 but found that driveability sufferred and there was no increase in fuel economy.

I almost forgot, there is a timing delay for Highway Mode which causes the mixture to "richen". It's supposedly for protecting the cats from overheating. I also turned that off/eliminated the delay which both increased mileage and had no ill effects on my cats. I drove over 40,000 miles and two years like that. However, I also drove a mixture of city & highway and our highways here have LOTS of hills. Lastly, I do a lot of passing/WOT so that may account for why I encountered no problems.

But, for a "pure highway" eprom (or if you have "stacked" bins on one of the newer flash proms), where you want max mileage for a highway trip, this may be something to consider.

I just bought my GTA, but on the way home i got 23mpg cruzing at 70mph
havent checked city yet im sure it will be around 15-18

Remember, elevation has a lot to do with gas mileage. The higher the elevation, the less air molecules. The computer will see this and lean out the fuel. I get around 20 in Ca. Drove to Idaho 2600 miles, 99 gallons (aprox 26 MPG). Most driving was over 4000 ft.

This is a good indication that your VE Tables are not properly tuned, especially with an SD car where the MAP sensor is a key component to selecting the correct cell in the VE Table (MAP vs RPM).

Ditto if you notice a large difference when the air temp varies.

with the combo in my sig I got 28hwy with the stock automatic PROM. I got 22-24 with mixed driving. 18 in town. not bad for a 300hp 305

what about timing? i was running at 9* BTDC now its at 6*. havent driven it enough to really see any difference. but is this a good idea or not?

My 82 with TPI350/4l60e/2.73 gets 27 MPG on the highway. Before the 4l60e upgrade she got only 17mpg. Thats an 10 miles per gallon inprovemet with the overdrive. This is all between 70 and 80 MPH.

I have had two 80's TA with that exact same driveline even down to the rear ratio. No matter how easy I drove neither of them got more that 21.

27 downhill with a hurricane blowing up your butt, maybe.

26 MPG from Chilliwack to Grand forks. This is passing some traffic and going through mountains. Car was loaded. Plenty of camping gear and one passenger. I definitely was easy on the throttle though going up hill though.

Stock 305 with 3" exhaust and 3.08 rear end.

700 km on a tank is pretty impressive Grim.

Four total 3rd gens in my life , two with 2.73 gears says that anyone claiming 24+ is full of it.

17 to 27 mpg with a transmission change, that's impressive. A lot of impressive numbers in here, I've heard a lot of great mpg numbers with 3rd gens, they are very slick, low cd, the birds more than the camaros. But how many are with the AC on? The new MPG standard that takes effect in 2008 will include a MPG with the AC on. My 20 - 21 MPG is at speed (70 - 85) with the AC on. Don't do much driving down here without the AC. I could try babying it in town to see if I can get the MPG up, but .................. WHAT FUN WOULD THAT BE!!!! I built this thing to FLY!!!!!!!!!!!!!!!!!!!!

I don't know if I would go that far. 2.73 gears can hurt more than help. besides I belive the window sticker for a 1989 350 formula claims 25mpg which for most is not going to happen but for others it will.

my old 88 350 tpi with a slp cam ,slp headers, 24# injectors 3.23 rear and a tpi 5-speed untuned got 25 mpg driving over 200 miles each day (as you can see it was easy to keep track of mpg)

How much tuning of the eprom have you done? Because I spent over a year working on it and trying every trick in the code (cause no one else knew anything about it). My results are repeatable (on a stock L98 SD car with an AFPR set to 50psi).

And my buddy how has an 89 MAF 383 stroker with a Miniram, AFR 195 Competition Ported 2.05/1.60 valves, 248/242 SOLID ROLLER, 1 3/4 SLP headers, Muflex 4" exhaust, 3400 rpm stall and 3.55 Moser 12 bolt rear end ROUTINELY gets 23+ mpg on the highway (and he doesn't baby his car). We are hoping to do a bit more tuning on his car and get it into the 10s at the track this year. But, because he doesn't have an EGR I have not invoked Highway Mode. but we are looking at converting to SD and I may try highway mode at that time.

Tuning is a KEY element that most people overlook. Unfortunately, most just concern themselves with WOT performance, but if they learn how to burn their own eproms and do a COMPLETE tune in all rpm & load ranges, they can easily improve both performance & fuel economy.

So I am now challenging you to learn eprom burning and try improving your fuel economy. The money you save in properly tuning your engine can easily pay for all of your equipment. I am confident that I can help get you into the high 20s on the highway. City will depend how much "stop & go" traffic you do because when you are just idling, you get 0 mpgs no matter what you do.

My 26 mpg was 100% highway. Its also what a 305 5speed is rated for on the highway.

I haven't really tried to work on A/C mpgs on my F-body as my A/C needs a Freon recharge and Freon has been banned in Canada for over 10 years

But, there are parameters for controlling the A/C compressor with the eprom on SD cars (not sure about MAF). I know on my daily driver (2004 Mazda 3 GT) that it gets the same gas mileage with and without the A/C on the highway.
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I am finding SD cars seem to get a bit better mileage than MAF cars; even without invoking Highway Mode. But it would be interesting to find two identiacal F-bodies (except one is SD and the other 89 MAF), optimize the Highway Mode and see if there is any difference in the mileage.

But you should also make sure that you are not quoting Imperial MPGs instead of US MPGs, there is a big difference. If a person get 10 kms/litre, that works out to US 23.5 mpgs and 28.2 Imperial MPGSs. The reason is because it takes 4.546082 Litres to make an Imperial Gallon while it only takes 3.78541 Litres to make a US Gallon.

But, before my eprom tuning, I use to routinely see 10.5-11 kms/litre on my L98 (when it was still bone stock) on the highway. That is 24.5-26 US mpgs. So I have no problem believing a 305 5-speed can get 26 US mpgs (11.06 Kms/litre).

damn thats just amazing Glenn!

That was on a trip I made from Kamloops to Calgary in the summer of 2001. And I passed every RV I got a chance too.

Hmm, it's been my experience that the 305 5 speeds don't seem to get any better mileage than the 350 autos. You'd think they would.

Grim Reaper, sounds like you need some Autofrost (R406a) to replace your R12 system. Cheaper than R134a and better performance than R12. Oil is the same stuff used in R12, so no problems converting.

Has anybody done before/after mpg testing with header installs?

When my L98 was bone stock, I got 18 MPG in the city. That was just with normal driving (maybe a couple hard accelerations, but no WOT). The car has 2.77's.

Now the car has Hooker 2055 headers/Ypipe, gutted airbox, AFPR set at 49 psi, and timing at 10 degrees. I'm going to have to do a MPG test this summer.

Ya, I bet your TA's did'nt have the 2.73 gear ratios. I believe the TA's came with 3.23. The rear end ratio alone would make a big differance.

Oh by the way, I'm talking new transmission, drive line, and well tune motor.

The car in my sig gets more then 20 MPG on combined hwy/city and when I run all HWY it will get 24 MPG.

I'm running 87 octane and I just got 223 miles out of a tank with my L98. This is with 80% highway and 20% city driving. I wasn't driving like a grandpa either. What do You guys think about my mileage?

so what about timing????? where should that be set?

I have a 1990 Vette, with TPI and a 6 speed. Last Nov I took a drive to my parents house and back (250 miles one way) with a little driving around while there. Tried to make it on one tank, but ran out of gas about a mile from home, right at the exit. The DIC said 29.9 mpg for the trip, but the actual calculation was 28.8 mpg. This is with the stock chip in the car. A '90 six speed vette has 3.33 gears, and 50% overdrive in 6th.

Scot

how much will a 180* thermostat affect fuel mileage

Depends on what you had before. I used to run a 160 stat and it pretty much always ran at 175 during the summer. Changed to a 180 and didn't notice much difference as I only went up 5F. Fan runs a bit less often now (on at 190 instead of 176).

I had a C4 Corvette guy with the digital instant read out tell me that at 180F, he was 1.5 mpg better than 160F. At 195F he gained another 0.5 mpg.

Back when I had the 160 stat, I tried some highway running on a cooler fall evening. It was cool enough that the coolant temp would go down to 160 and then back to 175....160...175. Doing 60 mph the entire way, I only got 20 mpg (instead of 22-23). 20 mpg is the same I got when I was doing the 80-100 mph average speeds in my post near the top. Running cold = burning more gas.

If you do no tuning (except the fans), your gas mileage tends to drop if you go to a cooler t-stat. But, if you properly tune, your gas mileage stays basically the same. In fact, if you properly tune, your performance remains pretty much the same too. Thus, with proper tuning, the t-stat has really no effect on either gas mileage or performance. I chose to run a 180* just because that is what I have used for over 35 years of driving. But I could have easily used the stock 195* or a 160*. The only problem with a 160*, you never really feel warm on a cold day.

I didn't do any testing on emissions so I cannot confirm what happens with a cooler t-stat and if you do proper tuning, if you compensate like you can for gas mileage & performance.

I'm going through about 14 gallons in about 260 miles average with about 50/50 highway and city....18.5 mpg in my lightly modded stock prom 91 Formula 350 with 100kmi.

With 4.5 gallons of gas I go about 45 miles in my all stock 305, I have no idea what the hell is wrong with it.

I averaged ~25 miles with 3.00 gears in my van on my TBI 4.3L, stock chip. For people that don't have em, an E-fan and a heated O2 sensor are a must. The heated O2 makes tuning easier, but when I was messing with my EFI it was amazing how much it WASNT active at lower RPM. The E-fan's got a less obvious advantage, my engine comes up to temperature quicker. I've been thinking about installing a block heater so when it gets even moderately cold out I could plug in and see if it gets me anything. Most of my driving is very short, within 10-15 minutes then it sits for a few hours a maximum of 30 minutes a day. Any thoughts on wether or not it would make a difference?

I could definately believe that. The Analog MAF sensor used on the TPIs is pretty crappy and has very limited resolution.

HI all,
The 27 report above is all highway. It was on a trip I did last summer going from Maryland back down to North-Carolina I uselly get anywhere from 16 to about 21 around town dependent on how I driving the car.

I do not have and L98 motor in my car, that why I call it an 350TPI. The motor is an GM 350 with TPI fual injection. The TPI injection system was tune for the engine. When I purchased the TPI system they ask me which engine I was using so they could tune it for the compression/cam profile. The motor and TPI system went into the car back in 1998. I was not surprise to fual the increase in MPG when I installed the 4L60e transmission.

i get 38mpg in my 88 formula its great!! did't i say its a feiro