Hot Rods rear gear

have a 32 highboy Roadster 350 Chev. 400 motor 350 trans. what would be the best rear gear to help fuel mileage. need to go further between fuel stops.. any help would be appreciated..John

 

not sure , what gear do you have in it now? what diameter rear tire?

 

How is the motor set up? Cam etc. What rear tires are you running?

 

2.56 to 3.08 depends on tires and cam.

 

Put an Overdrive in it.

 

I would think around 3.00 but it depends on the cam and converter. If you have a higher stall converter, you need to make sure your cruising speed is above that or the slipage and heat will ruin your fuel economy and tranny.

 

Right around 3.00:1, depending on rear tire size, however, we don't, as others have said, have enough information.

 

What kind of mileage are you hoping to get?

 

search project33.com and click on calculator

 

Changing rear tire diameter has the same effect as changing rear end ratio.

 

Not enough info, but generally 2.70 to 3.25 range are mileage getters.

 

Common wisdom used to be (and still probably is), to gear for peak torque at the desired cruising rpm. This is what auto engineers did unless it was purposely geared lower for a performance model, or to get a heavy small engined car moving better from a stop.

 

For a given speed it will take X horsepower to move the car at say 70 MPH. The slower you turn the engine the more torque it takes to make that power.

The slower you turn the engine the better your fuel economy unless you move the power valve.

Here is a chart showing a typical amount of fuel a 350 EFI Chevy burns to make 1 horsepower for 1 hour in pounds.



Practically speaking you want to keep the engine under 2000 at hiway speed or as low numerically as will fit the housing. Like a 2.29. Of course an OD would be a much better solution.

 

Depending on your combo (cam, compression, torque convertor, etc), you want to be doing about 2000-2200 rpm at 70 mph in top gear for max mpg.

Torquey low-reving engines would work better toward 2000 rpm (or maybe even a little lower). Smaller hi-winding engines would be better suited toward 2200 (or more).

Plug those values and your tire height into the project33 website calculator and it'll spit out the gear ratio.

FWIW, most modern passenger cars- with overdrive, EFI and computer-controlled timing- are turning about 1950-2000 rpms at 70 mph in high gear.

 

To make it easy, you burn 1.5 times the fuel at 3500 for the same HP as you would at 2000. So if you had 12 at 3500 you would get about 18 at 2000, or 15 at 2750.