Gas Mileage Gears
Gas Mileage Gears
(OP)
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.
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.





RE: Gas Mileage Gears
RE: Gas Mileage Gears
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.
RE: Gas Mileage 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.
RE: Gas Mileage Gears
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?
RE: Gas Mileage Gears
RE: Gas Mileage Gears
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).
RE: Gas Mileage Gears
RE: Gas Mileage Gears
Isnt horespower a function of torque
hp = tq * rpm / 5250
if so then if tq is lower than so must hp.
RE: Gas Mileage Gears
Maybe it is just the result of the way the dyno works and calculates both.
RE: Gas Mileage Gears
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.
RE: Gas Mileage Gears
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.
RE: Gas Mileage Gears
RE: Gas Mileage Gears
RE: Gas Mileage Gears
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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RE: Gas Mileage Gears
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.
RE: Gas Mileage Gears
I expect the Corvette or Pontiac will make over 400 HP and 10 HP will therefore be something like 2.5%
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RE: Gas Mileage Gears
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
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RE: Gas Mileage Gears
(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).
RE: Gas Mileage Gears
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.
RE: Gas Mileage Gears
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??
RE: Gas Mileage Gears
Cheers
Greg Locock
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RE: Gas Mileage Gears
Maybe if cars were shaped more like airplane fuselages, we'd have better mileage, and less pilot error?
RE: Gas Mileage Gears
RE: Gas Mileage Gears
RE: Gas Mileage Gears
RE: Gas Mileage Gears
Cheers
Greg Locock
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RE: Gas Mileage Gears
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.
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RE: Gas Mileage Gears
RE: Gas Mileage Gears
I'll bet the higher drivetrain speeds with the shorter gearing result in more drivetrain losses.
RE: Gas Mileage Gears
I wrote the following because I hear this all the time...
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RE: Gas Mileage Gears
RE: Gas Mileage Gears
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...
RE: Gas Mileage Gears
RE: Gas Mileage Gears
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
RE: Gas Mileage Gears
ISZ
RE: Gas Mileage Gears
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
RE: Gas Mileage Gears
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.
RE: Gas Mileage 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.
RE: Gas Mileage Gears
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
RE: Gas Mileage Gears
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.
RE: Gas Mileage Gears
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!
RE: Gas Mileage Gears
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...
RE: Gas Mileage Gears
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.
RE: Gas Mileage Gears
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] = [I] * [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 [I] (which had better be remaining constant).
Norm
RE: Gas Mileage Gears
Regards
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RE: Gas Mileage Gears
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.
RE: Gas Mileage Gears
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.
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