Gas Mileage Gears 1

[WickedWeasel]

I am not sure if I should post this in the gears section or the engine/fuel section, but I will start here.

the question is around gears and gas mileage. The discussion came up during a conversation about corvettes (which I happen to road race).

Corvettes from the factory come with 3.42 gears and 6 speeds manual tranmissions. Many corvette owners opt for 4.10 gears over the 3.42 gears because it lowers 1/4 mile times between 3-4 tenths. Everyone agrees with that which is great. The place that many disgree is fuel economy though. Many people say that fuel economy is not affected with 4.10 gears because of engine efficiency do to load on the engine while others state that fuel economy has to be affected since at 60 mph 3.42 gears are 400 less rpms than 4.10 gears.

Things I know

4.10 gears dyno 10 rwhp less than 3.42 gears
4.10 gears rpms are higher than 3.42 at the same speed in the same gear.

My thought is if the car dynos lower it then requires more energy to get it rolling from a stop. I also think that if a car is turning higher rpms with the new gearset then more energy is being used. More energy for starts and more energy for crusing in my little pea mind means less fuel economy.

Car tuners though are saying that with the 4.10 gears they see signs that the car has less load and thus they can lean the car out more. Because of that reasoning the car is running more efficient with the new gears and thus the fuel economy is not less, but the same.

so I decided to ask the people that really should know which are you guys.

 

[R1100S]

I wouldn't answer your efficiency question. It depends on too many factors. Speed you drive, BSFC vs. speed and load of the engine, when you shift transmission gears, etc. However, the 10 less rwhp is almost certainly an effect of bad dyno operation (miscalculation by the operator) and not a real world horsepower loss.

 

[WickedWeasel]

Sorry RWHP - stands for Rear Wheel Horsepower

We dyno tons of corvettes and all of them result in about 10 rear wheel horsepower loss when comparing 3.42 gears to 4.10 gears.

 

[dcasto]

Keep it simple. The reason for the vette having 6 gears is to get an overall ratio that lowers the rpms at high speed in order to reduce fuel consumption. You could have a 1 to 1 rear end, if the gears in the transmision were adjust by the 4.1 or 3.42. Think about FWD cars, they have no rear end gears.

So you are correct in your assumption on cars running at higher RPMs means more fuel, more energy, more acceleration.

What I can not see is that changing the gears affecting RWH. HP at the engine is torque times RPM, at the wheels, it's the loss of torque being consummed to accelerate the flywheel, shaft, and tires. I don't know how the WHP system works, that would be fun to learn though, does it use pumps and brakes? If not, then I agree with R1100s.

 

[ivymike]

My guess is that WW is using a chassis dyno, and is comparing two plots of horsepower (derived from the angular acceleration of a weighted drum which the drive wheels rub against) versus engine rpm measured by counting ignition pulses. Some of the horsepower difference might be due to "scaling" of the x-axis of one curve relative to the other, as a result of gear ratio differences. The same mistake was published in a hotrod magazine one time when they compared power output w/two different tire diameters.

My guess is that using the lower-rpm setup will result in slightly better fuel economy at cruise. The effect might not be large enough to be readily observable via single-tank fuel economy comparisons. Also, I wonder whether the speedometer and odometer are being corrected for the new gear ratio (presumably changed @ rear diff), or if people are cruising down the highway at lower speeds because the 4.10 gears cause the speedo to read high?

 

[R1100S]

Are the 4:10 gears factory installed or aftermarket. It may be that GM reprograms the ECM for less peak torque, and less peak power, with the lower gears installed. However, if you take a car with 3.42 gears, pull them out, and install 4.10 gears, you will not effect rwhp. Rear wheel torque, yes, but rwhp, no.

 

[WickedWeasel]

corvette speed is registered from the wheel speed sensor so changing the gears has no affect on the speedo. chaning the tire size would have an effect.

An Automatic though would require a change in shift points to prevent over-revving.

The dyno is an above ground chassis dyno and works as stated. It is in the process of being switched out to an inground load bearing dyno which allows our tuner to do the full tune by himself (all cars are tuned for the street and also tuned on a dyno for Wide Open Throttle - WOT).

 

[SomptingGuy]

Do you get similar differences in measured rwhp in different gears?

 

[WickedWeasel]

4.10s are installed aftermarket not OEM. Options for a 6 speed mn6 are only 3.42s (a4 have 2.73 or upgrade performance to 3.15).

Isnt horespower a function of torque

hp = tq * rpm / 5250

if so then if tq is lower than so must hp.

 

[WickedWeasel]

the 10 rwhp would be 3.42 to 4.10s. if you changed from 3.42 to 3.73 it would be less loss.

Maybe it is just the result of the way the dyno works and calculates both.

 

[dgallup]

Chassis dynos do not measure horsepower. Period. They measure rate of acceleration on a known inertia then throw in some fudge factors. You will always get bigger HP numbers with taller gears (assuming the drive train efficiency is constant) because there is a lower rate of acceleration and thus less HP "loss" in accelerating the drive train.

Taller gearing will normally give better efficiency because the engine will be running reduced revs at a larger throttle opening thus less frictional & pumping losses. However, in the case of the Vette with it's overdrive in both 5th & 6th gears the difference may be very small or negligible. In any case the Vette gets great highway mileage due to it's low weight, good aerodynamics, tall gearing and highly developed motor. It is the one GM vehicle I actually admire.

 

[WickedWeasel]

yes it doesnt measure hp I just talked to our tuner about this (this is why I am inside the car and not under the hood). The calculation of hp is rate of climb of rpm versus mph increase or something like that.

Based on talking to him I think I understand.

Fundamentally speaking a car that is running higher rpms with the same load will use more gas the caveat here is yes the car is turning more rpms, but it is now in a lighter load cell (do to the higher gear ratio) in the PCM mapping of load vs rpm thus the car requires less fuel.

He then went on to say some other jibberish that I could not understand - lol. Something about rpms outweighing load cell values and that affecting gas mileage.

I guess with modern cars you can play with so many variables that you can make cars very efficient compared to the old days of adjusting carbs.

 

[WickedWeasel]

BTW - thanks to everyone that responded. On other boards people attack you here I got answers without attacks and that makes it much easier to understand.

 

[DennisP]

I have a GMC truck that was just switched from 3.42 to 4.10 gears for improved towing performance. Even though the engine is turning 2500 RPM at speed on the highway instead of the 2000 RPM with the 3.42 gears, the gas mileage remained constant. In town mileage appears to be a little better. I can only attribute the change to the engine running in a more efficient portion of the power band.

 

[patprimmer]

Personal attacks and non constructive criticism are against the rules here and those who indulge in flame wars soon dissapere.

That is why those of us interested in discussing ideas and learning keep coming back.

I agree with what seems to be the consensus.

Here is my summation, repeating or expanding on some ideas already stated.

Same constant speed equals same load.

Very generally lower rpm equals lower fuel consumption due to lower pumping losses and less friction and less losses from inertia in reciprocating parts in the engine.

There are to many variables to make hard and fast rules as thermal efficiency and friction in the engine changes with manifold pressure.

Different gear sets do have different friction losses, but 10 hp sounds like a lot. These are to do with sliding action of gear teeth on each other and different torque and speed on gears on input side for same speed and torque output to the axle.

I don't know the individual gear ratios in a Corvette, but if the ratios are typical to those offered by GM in their gen 111 V8 powered cars here (Australia) then they are about right for highway cruise in 5th and overgeared in 6th and have no effective low gear (a critical point to me as I tow speed boats and need to go very slow up a steep ramp with extra load).

With the overgeared 6th gear, I expect a the shorter diff ratio makes little difference if all the upshift points are at the same rpm rather than the same speed as were done with the taller gears.

Changing final drive ratio can give interesting but wrong data unless both the speedometer and the odometer are corrected. Without correcting the odometer, with shorter gears, the recorded mileage increases by a greater margin that the fuel efficiency decreases, thus giving wrong but apparent increase in fuel efficiency. I once used this to effect when a company accountant was giving me a hard time about my running costs vs fleet average. Changed diff from 3.08 to 3.7 something. Suddenly my running costs were slightly higher in real terms but below average according to his data that simply divided costs by odometer reading. Simple solution. Everybody happy, unless he reads this.

Oh. front wheel drive cars do have a final drive reduction gear and differential. It is just located in the front rather than the rear axle assembly.






Regards

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Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[volpegrigia]

I believe gas mileage is tied to motive power. At the same speed, the same car should require the same motive power, regardless of gear ratio. The gear ratio will affect the delivery of power to the wheel and the efficiency of the drive train.
A Pontiac GTO with a 6 speed manual transmission (Tremec), a VI speed ratio of 0.57:1 and a final drive ratio of 3.46:1 and 17 wheels will have a motive force required of about 535N at 60MPH that converts into a power requirement of about 22.2HP. Knowing (or assuming) an overall efficiency of the drive-train one could find the needed fuel flow required and this the mileage. A Vette will probably have similar motive force and power requirements and the change in final drive could not possibly result in a 10HP (about 50% increase) of motive power.

 

[patprimmer]

Apart from OEM tests for fuel efficiency and emissions, few dyno tests will be done at anything less than WOT.

I expect the Corvette or Pontiac will make over 400 HP and 10 HP will therefore be something like 2.5%

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[GregLocock]

I can think of very good reasons why you might lose 10 hp in aftermarket diff with a higher ratio than the original diff- Efficiency - diffs are rather inefficient. An OEM diff is likely to have better efficiency than an aftermarket one, and a shorter diff's casing has to be stiffer for a given amount of deflection, which controls the misalignmenet of the gears and hence the efficiency.

As to the effect on fuel economy, it is incredibly complex. As a rule of thumb the longer diff would be expected to better, but as an example, when towing a trailer I know of one car that gets better (measured) mpg with a 4.1 rather than a 3.73, in certain circumstances. It depends very much where you are on the fuel map, which is controlled by your operating condition and the transmission strategy/ratios.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[globi5]

Why would GM choose a 3.42 ratio for the Corvette if it weren't affecting mileage?

(My guess would be that a relatively light car with low wind resistance driving on a highway with a powerful engine would generate significant pumping losses at higher rpms).

 

[WickedWeasel]

The 3.42 gears that Chevy uses has always been a debate. MY Trailblazer SS has 4.10 gears from GM and my road race vette has 3.73s from GM so it is not like they do not have choices.

I think everyone has to remember the vette is designed for highspeed Autobahn type driving (more like road racing than drag). 3.42 seem to be a good gear for that of driving. The problem though is top speed in a vette is done in 5th gear not 6th. The car just doesn't have enough power to get to redline in 6th.

As stated I run 3.73 on the track. I feel the 3.73 give me that little extra snap, but at the same time I can stay in a single gear much longer. With 4.10s I would have to upshift and with 3.42 I had to downshift too much.

With a 402 in a corvette I can run Full Watkins Glen in 4th and 5th gears which less shifts means less wear and tear and less chance to miss a shift. I can concentrate more on braking and driving.