Change gear ratio to improve mpg?

[Josborn]

Has anyone change the gear ratio in the rear end to improve the mpg for their vehicles? I was hoping to squeeze a few more mpg out of my 1935 Slant 48 to stretch past the current 9-10 mpg.

Thoughts?

[jimTN]

Less RPM equals less gas.................

[tubman]

Do you know what's in it now? The tooth numbers are stamped on the case (unless the actual grars themselves have been changed). A little math will give you the gear ratio.

[Seth Swoboda]

Yes. I have changed from a 3.78 to a 3.54 a couple of times and now I'm getting ready to change from a 3.78 to a 3.25 in a '37 Ford. You'll need to run the 15/29 gears (15 tooth main drive and 29 tooth cluster) in the transmission to make that swap more drivable out of 1st gear.

**Note** you will need to use a 39 transmission case and the 3in shift fork on your transmission cover to accept the 29 tooth cluster gear.

[Ol' Ron]

The stock or mild flathead engine runs most efficiently at 25/2800 rpm and this will tell you what you cruise speed is. Increasing the stock 6:1 CR to 8:1 will improve mileage as well.
Gramps

[Bored&Stroked]

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Less RPM doesn't always equal less gas - it depends on the load the engine is under and whether it can efficiently pull the gear ratio you're running at the speed you're trying to drive the car.

If you start lugging the engine where it doesn't have the torque and RPM to pull the gear, then mileage will tend to suffer.

[Seth Swoboda]

Originally Posted by Bored&Stroked

Less RPM doesn't always equal less gas - it depends on the load the engine is under and whether it can efficiently pull the gear ratio you're running at the speed you're trying to drive the car.

If you start lugging the engine where it doesn't have the torque and RPM to pull the gear, then mileage will tend to suffer.

I agree. I'm not sure if you can improve gas milage a "whole lot". Remember, we're using an engine technology that is 80-90 years old.

What is the condition of the subject engine? I would assume a freshly rebuilt engine with good cylinder psi would be more fuel efficient?

I tend to not worry much about fuel milage on my flathead engines. My focus is on a good running engine and lowering my RPM's at highway speeds to a comfortable point.

[cmbrucew]

Try larger tires.


Bruce

[ford38v8]

Let's do a little math. In simple numbers, you might get away with an expenditure of $500 to do the job, providing you do everything right and don't run into any problems.
You might then squeeze another 4 MPG out of your running gear, and perhaps fill the tank 5 times per year. At 14 gallons per tank, you would then have travelled an extra 280 miles in a year's time. Say you're now getting 12 MPG, and expect to get 16 after the conversion. At say, $4.00 per gallon, you've saved a whopping $70 in that first year alone. At that rate, you'll realize the return of your initial investment in less than 7 years! Such a deal, how can you go wrong?

[JM 35 Sedan]

Originally Posted by Seth Swoboda

Yes. I have changed from a 3.78 to a 3.54 a couple of times and now I'm getting ready to change from a 3.78 to a 3.25 in a '37 Ford. You'll need to run the 15/29 gears (15 tooth main drive and 29 tooth cluster) in the transmission to make that swap more drivable out of 1st gear.

Good information!
This is exactly what I did on my '35 fordor slantback sedan (avatar) several years ago. 3.54:1 rear gear ratio, 15/29 transmission gears from a '46-'48 side shift transmission. It drives very well around town in stop and go traffic, and out on the highways at 55 to 65 mph. The last time I checked gas mileage, it was ~ 18 mpg, but have since done some component rebuilds and tune-ups, so will need to re-check mileage.

PS...I did not make these changes specifically to improve gas mileage, although I knew it would. My main goals were:

1.) Lower engine rpm at highway speeds of 55 to 65 mph to prolong engine life. With 4.11:1 rear gears the engine was screaming at the speeds I wanted to travel. I spent ~ 6k$ rebuilding this engine in 2001 and and I didn't want to have to rebuild it again while I own this car. I've put nearly 40k miles on this engine since buying this car in 2007 and this engine still runs strong.

2.) The second reason for making this change was to improve shifting with the better 2nd/3rd syncros in the later gears and going to 15/29 gears that compliment the 3.54 rear gears. Also while making this change I completely eliminated the troublesome problem of popping out of second gear.

[35fordtn]

Id look into your carburetor and ignition timing. 9-10 is low way low even for a stock 4.11 geared car

[JayChicago]

Yes. This inquiry was also posted on V8 Club forum. Responses were: 9-10 mpg, something is wrong, fix that before replacing the differential. Suggestions were look for sooty plugs, send carb to Charlie NY.