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EFV-8 Club Forum / General Ford Discussion / Seeking advice on new rear gears

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Discussion Topic: Seeking advice on new rear gears

shadeboy

11-26-2024 @ 4:50 AM

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Posts: 17
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All - I have a 39 Ford Deluxe that is a hotrod. Thanks to input from this group, I have determined that my car has a Ford 9" rear with a 3.25:1 gear ratio. The current rear does not have a limited slip differential. I am working on a budget since I am already (severely) upside down on this car. I would like to install new gears that bring down my RPM's while cruising at about 60MPH without losing too much of my acceleration. I also would like to have a limited slip rear end. In the best situation, I would also be able to keep my current rear axle (housing) and just replace the guts. Any suggestions would be appreciated. Thanks.

juergen

11-26-2024 @ 6:32 AM

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You never mentioned what engine you have. With an open rear end I doubt if it is a flathead. Your 3.25 is about right for a 100 horse flathead for cruising. Lower (numerically) will not keep you rod "hot" unless you have a torque monster for an engine.

That said with your 9 inch, you can get any 9 inch pumpkin to replace your present one allowing ratios under 3 and limited slip options. On a limited budget, these are available at swap meets as most want the ratio above 3 and even 4.

shadeboy

11-26-2024 @ 7:08 AM

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I know this is sacrilegious so I was hesitant to mention it: I have a 383 stroker putting out 450 HP. When I got the car, it was well under way as far as the power train was concerned.

juergen

11-27-2024 @ 11:30 AM

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You still need to choose what speed and rpm you want. Now:

wheel speed in rpm = drive shaft speed in rpm/ rear end ratio R or with tread diameter in inches T

wheel speed in mph = (drive shaft speed in rpm/R) x (3.14 x T )/(12 x 5280) x 60

If the tire is 30 inches tall, and R is 3.25 and cruise rpm is desired to be 2300 rpm, then cruise will be about 63 mph.

You can use the formula to get the ratio you desire. For example if you want to cruise at 80 mph at 2200 rpm, then

R = (drive shaft speed in rpm)/(vehicle speed in mph) x (3.14 x T)/(12 x 5280) x 60 Again with a 30 inch tire diameter

R = (2200/80) x (3.14 x 30)/ (5280) x 5

R = 2.45

This assumes you have enough torque in first to get the car going and enough at 2200 rpm to overcome the drag at 80 mph..

This message was edited by juergen on 11-28-24 @ 7:07 AM

trjford8

11-28-2024 @ 8:23 AM

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Before you buy another 3rd member for that 9 inch you need to know the spline count on your axles. There are two spline counts 28 and 31. 28 is the more common. Moving to a 3.00 gear will drop your RPM about 150 . As explained above your rear tire size also has an effect on your RPM

This message was edited by trjford8 on 11-28-24 @ 8:26 AM

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