So I'm going to start commuting 200 miles a week and I was trying to figure out at what mph the highest mpg would be achieved. I am considering a bone stock tbi f-body. Factory highway gears, new 26" tires, fresh suspension, not slipping transmission, you know what I mean. With flat highways and the use of cruise control, what would you expect to net the highest mpg? I'm thinking somewhere around 65mph, but I'm curious to see what you think.

 

Do you have the stock 3.08 rear with the T5?

I have a fuel mileage modeler I will use when I get home. It was an old IC engines assignment that I love to use.

 

well if engine needs to do work maybe one should look peak VE in the tables. if that is max TQ and i am not quite sure on that i think it might be at that RPM and MAP cell. that may require you look at main fuel table. is this accurrate?

 

not sure if that post is accurrate. it looks like my tables might show 70 kpa and 1600 rpm. just opened up TC. seems like a good load on engine. 1600 rpm in 5th might be 60 MPH?

 

I have the stock t-5 and 3.08's. I figured the 700r4 and 2.73 gears would be about the same with the torque convertor locked. However the t-5/ 3.08 combo might be slightly higher.

I think you have the right idea Ronny, but wouldn't gearing, aerodynamics, ect. also play a part, not just engine parameters?

 

Actually I would think that a low load 30-50 kpa (map) would give the best mpg. Eventhough I have a caprice and highway mode when on a trip if I can keep it under 50 kpa (map) I get another 2-3 mpg. The highway mode adds 2-3 deg of time and runs at 128 blem. I'm still working on the milage (like 28 mpg on a trip) 3:42 gears in the mountinious north, Pa.
Just my 3 cents worth.

 

There are many factors that can influence total fuel economy. Using the model I generated back in school I have come up with a close apporximation. I will assume the following for this model

Driving in 5th gear (.63 for LO3 T5)
.34 CD (per tech data on TGO)
An ambient temperature of 77° F
Ambient pressure at 14.7psi
245/50/16 tires that are properly inflated with about 2% loss in overall height for compression with a rolling foot path of 1.047 feet
A curb weight of 3500 lbs
170hp at 4k rpm and 255lb ft at 2400rpm as max power
Static rolling resistance coeff of .008
Dynamic rolling resistance coeff of 7.4x10^-5
85% driveline effeciency (stab in the dark)
A viscous loss factor of the transmission of .042 (common value to use for envelope calculations)
26.2ft^2 as the frontal area of a 3rd gen (from data provided by kdrolt here
A road surface factor of 1 (assuming straight smooth road)
An estimated minimum BSFC of .4 (this value commonly used for FI motors but at road load conditions it can very up to .5)

Using these assumptions the theoretical fuel consumption should be similar to this

35mph - 33.99mpg
40mph - 32.64mpg
45mph - 31.15mpg
50mph - 29.59mpg
55mph - 28.00mpg
60mph - 26.42mpg
65mph - 24.88mpg
70mph - 23.39mpg
75mph - 21.97mpg
80mph - 20.63mpg

These numbers will be influenced by any changes in the above data. Some changes will be more significant than others. head wind, rough road, hills, tire compound, ect ect can change all of these numbers. The 15" RS wheels (215/65/15) yeild a 1% gain in mileage over the 16x8's.

 

You can also use the inj. pulsewidth if you have winaldl and can burn proms. Simply cruise along at each speed and get the PW. Determine the dutycycle and multiply by the flowrate of the injectors in pph, then divide by the speed in mph and you have your specific fuel consumption per mile. Obviously lower is better

 

dimented, does have a very exact way that I will eventually use.

ShiftyCapone, does your program take into account the torque curve of a v8? I don't know if that would make any difference, however just an added thought.

Ronny, you earlier stated 1600 rpm, but do to the limited visibility of the p3 and 8746, wouldn't that be all rpms between 1600 and 1975? That would be roughly from 60 mph to almost 80mph.

 

It is going to vary with engine load. My Audi has a trip computer and will give me current MPG consumtion (similar to dimenteds formula) based off the MAF sensor it uses. Lower engine speed is not always better, because the engine load gets is higher (IE, you have to use more throttle to keep it at speed.) I found that you typically do the best near the torque peak, because you engine load drops significatly compared to lower engine speeds.

 

My G20 does the best the closest it gets to its torque peak. I actually get better mileage going 75+ than 65-70. I have proven it with dataloging. It takes a lower pulsewidth to run 75 on flat ground than it does 65. Take the pulsewidth and multiply it by engine RPM and you will atleast be able to tell which is better.

This is just an example--Say I get 2 msec @ 1,600 rpm @ 60.
That would be 3,200
Now at 70 I get 1,800 and 1.5 msec. That is 2,700. See how much less fuel I am using.

The only thing that the RPM is needed for is to base the comparison off. Lets say I drop it to 3rd and run 75 @ around 2,700 rpm. Lets say the pulsewidth is 1.3 msec.
1.3x2,700=3510. Third gear is wasting gas. If I was pulling something the load in OD would give a longer pulsewidth and waste fuel.


On my G20 I found that it was most economical to keep it from going into OD around town until 45 mph but lock the converter in 3rd going about 35.

 

Keep in mind that you ahve to calculate teh duty cycle, which is a function of 1/rpm. The duty cycle is what the fuel flow is a function of. Also dont forget to take speed into account. More fuel doesnt always mean less economy. If your going faster it may mean youll go farther on the fuel you use.

 

ShiftyCapone, an engine's BSFC curve starts off high, goes low, then returns high as the internal friction starts to build. For this reason it's almost impossible for the majority of people to figure out their mileage.

As the speed increases so does the aero drag and anything over about 65mph starts to become a real aero drag (pun intended). Rolling resistance of the tires, bearings, and powertrain inertia are the major factors below 65mph.
Scientifically speaking the best mileage would be the slowest you can go with everything else optimized. This is because aero dynamic drag would be almost zero. But you wouldn't get anywhere soon...
For max mileage I cruise just about every vehicle at 65-70, anything higher and the aero starts to get really high up until your top speed.

The lower your engine speed the worse your city mileage might be, same with highway if you can't get into the engine's sweet spot. The sweet spot is of course the lowest BSFC rpm. Most v8's have that area between 2000-3000. Everything bolted onto an engine that flows some gas determins the BSFC.

 

It returns high at part throttle cruise? I know it goes from high to low but am unfamiliar with it returning to a high value during part throttle steady RPM runs.


The power value for the RPM the vehicle opperates at a certain speed is calculated and used in the model. There are a few ways to model this and the most basic way is like 15 steps. I bet you could find those steps on the net somewhere.

JP hit it on the head. Basically. your best milegae is in top gear with as little RPM as possible. However, as JP noted, you won't be going anywhere fast. What you end up with is a car that averages all these evils (power, mileage, drag etc etc) into something that is average in all these categories.

 

I think he means if your cruising along at 140 at 4000 rpm with your foot to the floor. Yeah, all engines get inefficient as you go to higher and hogher rpms.

 

Parts of it does, but the VE of the engien goes up to a certain point. I would bet my reputation, that the BEST mpg is not at a very low RPM, as the engine is not very effeciant there. Just like in my Audi post, Most of the time, at legal freeway speeds, my Audi gets its best milage in 4th gear, not the same speed, but in an overdrive gear.

Now, emmisions testing and so forth, I would bet that in stock form, these cars are designed to acheive the best milage somewhere near the legal freeway speed limit in most states. It would only make sense for GM engineers to pick gearing that would allow this (not that GM does things that actualy make sense).

 

Shifty, sorry, I ment with engine speed. Also, BSFC doesn't follow the VE curve. Friction isn't included in the VE, neither is internal windage (pumping losses), and inertial losses (keeping a heavy rotating assembly rotating).
So at 1000rpm the BSFC might be .7, dip down to .4 at 2500, and then start to climb back up beyond that. The motor might have peak VE at 3600 but 2500 is the sweet spot for that motor.

For a smaller engine like a 4 banger or small v6, the engine rpm for best mileage might be closer to their peak VE or maybe even above. It all depends on how much of those "other" losses affect the engine.

Yeah, GM might not do some things "right" but they actually are. They just have a lot more things to weight out, CAFE, emissions, horsepower, drivability, durability, all of those things play into their design so you'll find lots of compromises. CAFE is becoming increasingly important to consumers... when will GM start making a turbo small displacement v8 with 4-cyl mode? Who knows. Hopefully soon because gas prices sure as hell aren't coming down.

 

I gottcha now. That clears things up.

 

no theory... but alot of driving with my 91 vert when it was stock:

around 65mph...

 

i have nothing to support it but on my 1850 mile trip from school to home i averaged about 75-85mph and seemed to get 27mpg consistently. my auto was in OD and the engine was at about 2200rpm give or take a little.

 

That sounds about right. You'd probably get metter if you held it around 70-75.
With the 3.73's and stock size tires I'm at 2800rpm at 75mph and get 23.5mpg. I get my best mileage at 65 but I don't like getting passed by 18 wheelers so I'm around 74 on the major highways. At 65mph I managed 25mpg.

 

Sorry to bring back my own thread from the dead, but now that the EBL is out this question should be easy to answer. Has anyone had enough play with it to make any strong judgments?

 

Hey, we might not be the fastest third gens, but we sure use less gas. God bless TBI

Oh, I am gonna second 60mph. I feel like I get the best mileage if I run it around 60.

 

75 MPH gives the best instant/average readings on the EBL for me.

 

Fast,

How does that compare with the RPM where your peak torque, and peak VE occure?

 

I am running about 2,200 RPM @ 65 MPH and about 2,400 @ 75, with around 50-80 MAP (varies with load) with a wideband of 15.8:1 and about 35* timing advance. GM rates this engine for a peak torque of 320 ft/lbs @ 2,400 IIRC. I have added some bolt-ons that have increased the low-mid range torque greatly. I am probably making around 350 ft/lbs @ 2,600 (peak VE) or so That puts me near peak torque and in a pretty high spot of the VE table.

Please excuse the VE table, I still need to work on smoothing the areas that I have been unable to hit with the VE learn.

PS. This is a highway run with the cruise set at 75 from the time I left Ft.Worth to just on the other side of Hillsboro. This was where the first stop was made and the laptop put away. On the trip to Padre, I averaged just over 20 MPG for the whole trip, doing 75 most of the way.