I've been fooling around with Excel, just looking at rear gear changes and effects on cruising engine speed. If one were to change, on a real car, a rear end from 3.23 gears to 3.91, which is a 21% change, would mpg's go down by 21% or would it be more like 10%? I know that the engine would be turning 21% faster, therefore pumping what is hopefully around 14.7:1 air/fuel mixuture through the motor 21% more often (or faster); but since the work output of the motor is the same (assuming say 55mph cruising speed), would mpg's have to suffer lock-step with such a gearing change? I know torque peak has to come into play somehow, so I'm guessing it may be more than a 21% hit for a slant with a stock cam (torque peak at 1600 or so), but just asking in general.

Short answer: I am guessing you would find a greater than 21% mileage drop, in the instance you pose.

Long answer: Assuming a constant mix of city/highway driving, and keeping tire size fixed (let's say at any standard normal street-driven passenger car tire size), I think you would be able to plot a bell curve of fuel mileage if you were to test all the available rear axle ratios from 2.26 through 4.56. The wind and rolling resistance factors vary only with road spead, not engine speed. However, fuel consumption is variable with engine speed/load. With the taller gears (low 2.xx) the engine would be lugging (below its torque peak) all the time, so you'd have your foot in it all the time, dropping manifold vacuum and increasing fuel consumption. With the deeper gears (high 3.xx, 4.xx), you'd have the throttle way open all the time to keep your road speed up, dropping manifold vacuum and increasing fuel consumption. With moderate gearing (mid-high 2.xx to low-mid 3.xx) the engine's torque peak would overlap the engine speed range most commonly encountered in your mix of driving, minimising fuel consumption.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

To me this dosen't make sence. With deeper gears the car is easier to push and therefor less throttle.
I do think that economy will drop, but I don't think more than 21%

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67Cuda,FAST EZEFI,340cu,CR=10.25,RollerCam&Rocker (XR268HR,#20-810-9)(#1622-16)(EddyRPM#60779,#7576), (MSD6AL,#6425) A904, GearVendorsOD, 8 1/4,3.55:1, ClassicAir

Easier to push off the starting line, yes. But the engine has to turn many more times a minute to run at a particular speed down the road.

Every time that engine turns, something has got to fill those cylinders...

-Mac

The car is equally difficult to push regardless of gears. Physics: It takes the same amount of work to accelerate a given mass to a given speed with given wind resistance and up a given slope. If you scale one 4-foot step, you've done the same work as if you'd scaled four 1-foot steps.

Engine RPM is equal to road speed divided by tire circumference times final-drive ratio times transmission ratio. With a deeper (higher numerically) final-drive ratio, engine RPM is going to be higher for any given road speed, which means the throttle has to be opened wider to maintain the engine speed needed to maintain the road speed.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

If this a stock or lightly mod car for the street only I would stick with the 3.23 gears. Been there done that. You will be more happy than what a 3.91 will pick up on a stock engine. Thanks Ron Parker
PS when my race car was a street car and it had a stock Slant in it with 3.23 gears I blew the chrome around the windshiehd off it running 105 mph on the Interstate 75 on a correct speedometer. That dawg was still trying to hunt to.









Bagels Juiced Im Bamboozled

swapped a friend a 2.76 rear for a 3.23 rear-- he wanted hiway gears-- at

70 mph his rpm dropped 450 between the 2 gears--- did not ask him mpg

question-- just FYI from real world application.

By the way his car is a 1963 valiant ragtop

This is my 69 Dart it was stock except the 3.23 7/14 rearend. But this is what was cool. I could race my stock Dart on the 1/8 drag strip and manualy shift it from first gear to second gear and it would run fine in second gear thru the 1/8 mile about 5000 rpms. Then drive it home at 75 mph. Damn drag racing was a lot of fun back then. Thanks Ron Parker







Bagels Juiced Im Bamboozled

I'll bet you this. Car in park or neutral. You hold your gas pedal to the floor and I match your engine speed with no more than 1/4 thottle, I will burn less gas at the same RPM.
Yes engines that turn higher RPMs will burn more fuel, but its not 1:1 (rpm:fuel) A lower gear (4.10) is easier to turn both from a start and while crusing and therefor requires less throttle opening and therfore less fuel. Example:
2 cars 70 mph, slite head wind, and up slite grade.
1car 2.76 gear, 2500 rpm, requires 1/2 throttle to maintain speed.
2 car 3.55 gear, 3200 rpm, requires 1/4 throttle to maintain speed.
Now I am making some asumtions here but the theroy makes sence to me that car 2 would need less throttle than car 1. If car 1 were running the same rpm as car two, one might assume that car2 would use half as much fuel. But car1 is not turning the same rpm and therefor should save some fuel simple based on the efficency of not turning as fast (friction, ect). The kicker here is that car1s rpms are not half as much it turning only 22% slower.
So based on the rpm car1 would only be able to save a max of 22% but it throttle is open twice as far. Now I know the math if rough but I sermize that there would only be an 11% loss in fuel mileage by switching from 2.76 - 3.55 gears.

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67Cuda,FAST EZEFI,340cu,CR=10.25,RollerCam&Rocker (XR268HR,#20-810-9)(#1622-16)(EddyRPM#60779,#7576), (MSD6AL,#6425) A904, GearVendorsOD, 8 1/4,3.55:1, ClassicAir

3.91s will not help with acceleration anyway on a stock motor. I had 3.00 gears in my Valiant and ran about the same 1/8th mile times after I put 3.55s in. I have heard many others report this too.

IMHO, you should not lose 21% MPG by going from 3.23 to 3.91. Wind resistance is the biggest mileage factor at highway speeds. This has been my observation with several cars.

If you have 2.76 gears now, going to 3.23s is a great swap and you will be happy with performance and mileage.

Lou

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Home of Slant6-powered fun machines since 1988

And how, exactly, do you intend to do that?

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

This surprises me, but I'll defer to your experience.
Wind resistance is by far the largest factor but you also need to consider internal losses in the engine itself. The nature of friction losses are that they increase exponentially with speed so that internal friction is much higher at high RPMs that it is at lower rpm. This is a significant factor to consider and it depends entirely in the speed of the engine whether moving or standing still.

*EDIT* And don't forget parasitic losses from external accessories such as the alternator, water pump, etc. These also increase exponentially. If you double the engine speed you will approximately quadruple these losses.
Yep, I believe that!

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David Kight
'62 Valiant Signet, White
'98 Dodge Dakota
'06 Jeep Liberty

Growing older is unavoidable but growing up is strictly optional.

I've been keeping my eye open for an early 80's D100/D150, and it just seems to me that a 4,000lb truck ought to need more gear than 3.23--especially if it ever got loaded up.

Since I don't actually own one, the whole excerise is just theoretical. I was just wondering, if I finally bought my truck, and found the acceleration a bit underwhelming, and did a rear swap, would a 20% deeper rear automatically make my 20mpg go down to 16mpg, that's all. It sounds like that, possibly a bit worse.

My '89 D100 longbed has a 318 optimistically rated at 170 hp and 260 lb-ft. It has 235/75R15 tires and 2.76 gears. It's OK empty, and a little slow but tolerable when loaded. 2.94s would probably be perceptibly better, and 3.23s would probably be just about perfect for it.
Ah! No. Depending on your starting and ending ratios, and the power and torque characteristics of your engine, you'd probably pick up some MPG in the city and lose some on the highway. Here is a chart from the Petersen book I'm always recommending, showing the effect of rear axle ratio on fuel economy in early-'70s full-sized passenger cars with 390 and 420 CID V8 engines and 3-speed automatic transmissions. These show better fuel economy across the board with a taller (lower numerically) final-drive ratio, but it's also not a slant-6...that's where engine characteristics come into play. This what we're looking at is the upward-sloping side of the bell curves for these two cars, and we'd probably never see the downward-sloping sides of the curves 'cause we're dealing with very large engines with very strong low-RPM torque, and 2.29 is approaching the practical limit of the ratio that can be achieved in a hypoid rear axle sized to fit under a passenger car. If we were to introduce a fourth gear (as for example with an auxiliary overdrive unit or an overdrive automatic transmission), we'd likely then be able to see the downward-sloping sides of the bell curves. You can divide the numbers (acceleration, fuel economy, ratio) and see how the percentage drops don't always match up.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Thanks Dan. I find it interesting that a 28% reduction only caused a 14% reduction in highway mpg's. Of course, you're right, it seems that as one goes over the other end of the torque curve, the mpg reduction would pick up. And, those numbers at 50mph, not a more typical 60mph backroads speed (at least around here). mpg is important to me, a big chunk of the justification would be a spare vehicle in case one of the daily drivers conks out, and I need to make the 45 mile trek to work.

I did find a truck locally, I'm guess it's not running (air cleaner is off) but the photo made it look good (no rust shown on the entire left side). But it's an auto, which makes me worry a bit, in terms of performance under a moderate load, and ultimate mpg's unloaded. I think '85 would have a lockup convertor, and deeper (2.54) first gear; but the rear gear could go either way (3.23 or 3.55), probably 3.23 for a short bed. I think 16mpg was the EPA estimate; I'd guess at best I'd pull off 18.

Temptations, temptations ... If only I had a concrete need for a truck!