Gear ratio advice
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Freshman User
Joined: May 2009
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From: Santa Fe, TX
Gear ratio advice
I have a choice of 2-9 inch rear axles to use. One has a 3.25 gear and and the other has a 3.70 gear. I will be running a 302 with a C4 automatic transmission. I was wondering which rear axle ratio would be better to use in my 52 F-1. The 52 will be a driver (hopefully daily if I can). Thanks in advance.
Adam
Adam
Cargo Master
Joined: Jun 2001
Posts: 2,571
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From: Oxford, Indiana
The 3.25 usually works out the best without an overdrive gear. It should put your cruise RPM's pretty close to the sweet spot with most common tire sizes. The 3.70 would work fairly well with an overdrive.
Cargo Master
Joined: Mar 2001
Posts: 2,759
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From: Elk River, MN
Here is an old rule of thumb for RPM at highway speed in high gear.
RPM= MPH x axle ratio x trans ratio x 336 / tire diameter
RPM@60 = 60x3.25x1x336 divided by tire diameter
RPM@60MPH w/28" tire= 60x3.25x336/28
RPM@60MPH w/28" tire in high gear w/3.25 axle =2340
RPM@60MPH w/28" tire in high gear w/3.70 axle =2664
I probably got his formula from John Nolan about 10 years ago.
RPM= MPH x axle ratio x trans ratio x 336 / tire diameter
RPM@60 = 60x3.25x1x336 divided by tire diameter
RPM@60MPH w/28" tire= 60x3.25x336/28
RPM@60MPH w/28" tire in high gear w/3.25 axle =2340
RPM@60MPH w/28" tire in high gear w/3.70 axle =2664
I probably got his formula from John Nolan about 10 years ago.
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