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.

 

[LateApexEE]

"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."

You mean Dynojet Chassis dynos (and other inertia dynos) do not measure HP directly. Not all chassis dynos use this method. In fact many modern, quality chassis dynos use very light drums and power absorber of some sort (hydraulic pump or eddy current) with a load cell to actually measure force applied to the rollers.

While I don't have anything useful to add to this discussion on gearing I thought I should clarify this misinformation.

 

[izzmus]

Real-world data in an RX-7 showed no difference in fuel economy between 3.91 gearing (factory) and 4.78 gearing. This was correcting for speedometer error, of course. Driveshaft harmonics were brought down into highway-speeds range, unfortunately.

I actually did see a slight economy gain with the 4.78's, but this may be because I switched to synthetic lubes in the transmission and differential at the same time.

This was with a carbureted engine. With my experiences with fuel injection, lower RPM would potentially make for better economy simply because it allows finer control over the fuel injection.

Not having any valvetrain loads certainly couldn't have hurt, either!

 

[izzmus]

I forgot to add: The early models had a choice of either a 4 speed or a 5 speed overdrive. The four speed was the same transmission as the 5 speed, just the 5th gear was replaced with a blanking sleeve. The EPA fuel economy rating for both transmissions was the same.

Given that top speed was also achieved in 4th gear, I assume that the only reason that they even bothered with an overdrive gear was to placate people who thought that driving 4000rpm to keep up with traffic was a little absurd.

The overdrives got serious when they started using fuel injection...

 

[RossABQ]

I'm not aware of FWD's that use planetary gears for the final drive; admittedly I haven't had my head into every model. All the ones I am familiar with use a helical gearset at the differential. Am I wrong?

Ditto the positive comments on the wide-ratio (vs. close-ratio) Tremec 6-sp's; any "fun" you could have in 5th and 6th would be at speeds not common even in the wide-open Western US, 4th will take you to 100. The economy and quiet operation you get in 6th on the interstates is much more valuable. 5th is perfect for loafing along on typical inter-city routes @ 50 - 60 mph.

 

[NormPeterson]

"You mean Dynojet Chassis dynos (and other inertia dynos) do not measure HP directly. Not all chassis dynos use this method. In fact many modern, quality chassis dynos use very light drums and power absorber of some sort (hydraulic pump or eddy current) with a load cell to actually measure force applied to the rollers.

While I don't have anything useful to add to this discussion on gearing I thought I should clarify this misinformation. "
Very light drums don't have anything to do with it. Whether acceleration is being computed from the change in rpm over time against a known inertia or from a net force applied at the tire contact patches still does not account for rotational inertia effects and friction losses within the vehicle. External force measurement methods can't "see" what's happening upstream.

Rather crudely, [Torque] = * [alpha] means that in the lower gears when you have a larger [alpha] due to the greater mechanical advantage the engine has over the load you necessarily waste more [Torque] accelerating the (which had better be remaining constant).


Norm

 

[patprimmer]

On a brake type dyno, weight of rollers does not matter as the power is measured at steady staten si inertia in the rotating parts does not apply.

Regards

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[Warpspeed]

My own personal experience with changing axle ratio agrees with DennisP's observations in an earlier post.

Assuming you are driving reasonably conservatively, a higher numerical gear ratio need not seriously reduce fuel economy.
This assumes typical city/suburban driving where vehicle speeds are low and constantly changing. A shorter gear gives more acceptable acceleration with much smaller throttle openings. This would be at relatively low speeds where most of the developed power is used for gentle acceleration to keep up with the general traffic flow.

If the vehicle is geared taller, although engine rpm is reduced, throttle openings will need to be wider and throttle "open" times longer to keep up with traffic flow. It is debatable if total engine airflow is actually less with a much taller gear when you demand a certain rate of acceleration.

Out on the open road, where road speeds are much higher, excessive constantly high engine rpm will definitely hurt economy, but around town this does not appear to be the case.

My advice would be if you drive mostly around town in heavy traffic, go for the 4.10 gears. If you do a lot of long freeway trips interstate, the 3.42 gears would be my choice.

 

[Tmoose]

When the OD relay fails on a typical 80s Volvo 240 the mpg can be counted on to drop 2 or 3 or 4 mpg. OD ratio ~ 0.7, with non OD ~ 20 mph/1000 rpm.

Efforts to keep close to torque peak are not really justified these days. Engines with computer FI managment even withoug variable valve timing have torque curves that extend way down there.

http://www.rockcrawler.com/reviews/toyota/4runner_2003/torque_curves.gif