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.

 

[chucksk]

As you know, the engine has an efficiency curve and a power curve. The RPM where the peak efficiency occurs you would want to correlate with the highway speed you normally drive at to get the best highway mileage. This would take some testing on the dyno & comparing to recorded highway speeds with the car. Then choose the gear ratios that best correlate with peak efficiency at that speed.
But, even with the nice slipstream of a corvette, it's still air resistance that eats mileage, and it's a cubic function (meaning going 70mph eats up a ton more of energy than going 50mph.) If you drive a little slower on the highway and get the gears that best fit this average crusing speed, then you'll see significant energy savings. For most vehicles, the peak fuel efficiency occurs between 40 and 55 mph. Owing to shape, the corvette is probably more on the high end - suitable for 55 mph roads. (boxes, like the typical SUV's on the roads today, push a lot of air, so they need to go wayy slower to find their peak efficiency speeds).
My best guess is find the ratio that works best at a slower average highway crusing speed; drive that speed and save the fun for one day a week??

 

[GregLocock]

Actually in fuel consumption terms it is a square law.

Cheers

Greg Locock

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

 

[chucksk]

Thanks Greg! Still lots of power to push at higher speeds..

Maybe if cars were shaped more like airplane fuselages, we'd have better mileage, and less pilot error? ;-)

 

[volpegrigia]

Wind drag force is a function of the square of the velocity. Wind drag power is a function of the cube of the velocity. Thus going twice as fast will require 8 times the power.

 

[dgallup]

But you get there in half the time.

 

[SomptingGuy]

... thereby saving fuel of couse. :-D

 

[GregLocock]

Well, yes, I had assumed that the purpose of transport was to get from A to B, rather than competing to see who could burn a mass of fuel in a given time.



Cheers

Greg Locock

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

 

[patprimmer]

Greg

I think you are half right.

The purpose of transport is to get from A to B at a reasonable time and cost. Any trade of between time and cost can usually be best assessed by the individual involved.

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.

 

[Bribyk]

As a rule of thumb, most engines achieve their best fuel economy at an RPM corresponding to a piston speed of 5 to 6 m/s (16.4 to 19.8 ft/s). Piston speed (ft/s)= 2*stroke(inches)*rpm/720.

 

[kevindurette]

"What I can not see is that changing the gears affecting RWH."

I'll bet the higher drivetrain speeds with the shorter gearing result in more drivetrain losses.

 

[kevindurette]

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

I wrote the following because I hear this all the time...

http://kevinthenerd.googlepages.com/torque_vs_hp.html

 

[SpinningDorito]

In my Rx-7 I get the best highway mileage at 60 & 80mph. This is at approximately 3000rpm and 10inHg of vacuum in 4th and 5th gears respectively. My instant injector duty stands right around 20% at both speeds. Because of it's low torque at low engine speeds i have 4.10 gears OEM. For us it's a switch to 4.33's for the 1/4mile. In any case, i've found that i get the best mileage at the point where my injector duty & rpm is lowest (duh), and vacuum is highest. If you have the facilities to install a boost/vacuum (vacuum in your case as i suspect you're naturally aspirated) gauge for the purposes of fuel economy i would recommend it.

 

[kevindurette]

SpinningDorito, if your RX-7 is a third-gen, you have a relatively low drag coefficient and an engine that hates to loaf/lug. The induction is quite inefficient at lower engine speeds. Anyway, these two effects combine to provide a best mileage at those higher speeds.

I've considered putting taller gears, milder cams, and a stand-alone management system into my Saturn ION. That way, I'll get the benefits of taller gearing (less engine drag) without the problems of taller gearing (engine inefficiency). Since I love electronics and control systems, I'm pretty confident in my ability to put together and tune a Megasquirt, but I can't find gears short of CADing them myself and going to a machine shop. I wonder how long it would take driving the car in order to make up such a cost...

 

[SpinningDorito]

It is a third gen, it's easier to get the better mileage at 60 because at 80 just about any hill or acceleration makes me go in to boost. If you're going to build/tune a megasquirt you should be able to get pretty great mileage at steady state cruise. Our engines aren't known for fuel efficiency but i know of some second gen n/a guys getting upper 20's to the gallon. I imagine if you tuned with a wideband and pulled all the fuel you could while maintaining a margin of safety you should make similar or better numbers.

 

[Metalguy]

Put some thermocouples on the diff. and you'll see where the 10 HP is going. A 1-1 diff. gear would run much cooler.

GM, and other American manufs., don't appear to understand how gearing "should" work. The gaps between gears (as a %) should become less in the higher gears (look at motorcycle gearing). The Vette has a HUGE gap between 5th and 6th, and a big one between 4th and 5th.

So, you're driving in the mountains and want to pick up a few revs, as you would do with a close-ratio box. But even with 6 gears you can't do that. Might as well stick with the awfully geared automatic trans., with it's huge diff. between 3rd and "OD".

Now a Doug Nash/Richmond/whatever 5 speed, THAT's a manual trans!!!

Gold is for the mistress - silver for the maid
Copper for the craftsman cunning in his trade.
"Good!" said the Baron, sitting in his hall
But iron - cold iron is the master of them all.
Rudyard Kipling

 

[IceStationZebra]

Kind of off the subject, but way in the beginning dcasto said "Think about FWD cars, they have no rear end gears" Of course they don't have "rear" end gears but unless their design has changed in the last 10years they do have a final drive gear set.

ISZ

 

[Metalguy]

FWD uses a more efficient planetary gearset instead of a hypoid set which has a lot more tooth sliding.

Gold is for the mistress - silver for the maid
Copper for the craftsman cunning in his trade.
"Good!" said the Baron, sitting in his hall
But iron - cold iron is the master of them all.
Rudyard Kipling

 

[kevindurette]

"GM, and other American manufs., don't appear to understand how gearing "should" work. The gaps between gears (as a %) should become less in the higher gears (look at motorcycle gearing). The Vette has a HUGE gap between 5th and 6th, and a big one between 4th and 5th."

For racing and for pure performance, I agree 100%. Your priority should be in the higher gears where usable horsepower is optimized. You can't put full horsepower to the ground early on; you'll lose traction.

However, I like what GM has done. The first few gears are where the fun is, and the last gears are for the daily commute. I really like this Dr. Jekyll / Mr. Hyde approach. It's the best way to have the same car for the road and track. If you're actually finishing 4th gear and getting ready to shift to 5th on a wide open throttle, redline to redline run, I really hope you're either on an open deserted highway or on a sanctioned race track. With six or seven liters (depending on your model), I think it's stupid to NOT have such a high 6th gear available for mild steady-state cruising. I wish I had such a gear.

 

[curtis74]

WickedWeasel wrote: "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."

I think you'll find that most owners switch to the 4.10s because of the T56's very high gearing. GM (as has been mentioned before) tends to miss the mark on gear ratio mixes. T56 gear ratios start at 2.66 for first and end with a double OD .50:1. Contrast that with the 4L60E automatic offering ranges from 3.06 up to .70:1.

I'm currently swapping the 4L60E in my 96 Impala to a T56. In the process I'm going from the stock 3.08 rear to 4.10s and keeping nearly the SAME overal final drive ratios.

4L60E with 3.08 = 9.42 first and 2.156 OD
T56 with 4.10 = 10.9 first and 2.05 OD

Power shouldn't be affected by gear changes. Minor discrepancies might be noticed, but 10 hp is pretty optimistic.

MPG is a tough one to pinpoint. For any combination there is a "best" cruise RPM. Lowering highway RPMs won't always return better MPG. Sometimes raising it helps by preventing lugging the engine below the torque peak.

 

[evelrod]

M---
As to all FWD using a "more efficient" planetary, etc.,etc.
How about my Austin Mini? It's certainly a FWD and it uses a simple straight cut crown wheel and pinion with a straight cut four synchro gear set. I'd be hard pressed to find a more efficient method of power transfer in any other FWD vehicle.

Rod