Gear Ratio

A quick formula to determine gear ratio given a set tire size.

Tire Diameter / 8.5 = gear ratio
(given a 1:1 final drive at the transmision.) to run 2375rpm @ 55mph
 
Lets say you decide on a 40" diameter tire for your off road truck.
40/8.5 = 4.7058
So a gear ratio of approximately 4.7:1 would be an appropiate ratio
for the front and rear differentials.(4.88:1 would be an available gear set)

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            Of Gear Ratios, Tire Speed and Diggin Mud
                           by Swamp Rot

              Tire O.D. x 3.14/12 = (ft per rev)
              5,280/(ft per rev) = revs per mile
      6000/ratio x 60/(revs per mile) = tire mph @ 6000rpm

38 inch tire = 531 revs/mile
34 inch tire = 593 revs/mile
32 inch tire = 631 revs/mile

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so some examples

38 Inch
3.07 x 2 (low range) x 2.52 (1st) = 15.47:1 | 43.8 mph @ 6000 rpm
                       1.52 (2nd) = 9.33:1  | 72.7 mph @ 6000 rpm
                       1    (3rd) = 6.14:1  | 110.4 mph @ 6000 rpm

(7.8 sec 11th/20 N class shift to second 3/4 7/8 down track)
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34 Inch
4.10 x 2 x 2.52 = 20.664:1 | 29.37 mph @ 6000 rpm
           1.52 = 12.464:1 | 48.7 mph @ 6000 rpm
           1    = 8.2:1    | 74.0 mph @ 6000 rpm
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34 Inch
4.10 x 2.52 = 10.33:1 | 58.7 mph @ 6000 rpm
       1.52 = 6.232:1 | 97.4 mph @ 6000 rpm
       1    = 4.1:1   | 148 mph @ 6000 rpm
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38 Inch
3.07 x 2.52 = 7.736:1 | 87.6 mph @ 6000 rpm
       1.52 = 4.66:1  | 145.5 mph @ 6000 rpm
       1    = 3.07:1  | 220.9 mph @ 6000 rpm
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comparisons and examples

                            38 inch    34 inch
4.56 X 2.52 = 11.49:1     | 59 mph   | 52.8 mph @ 6000 rpm
       1.52 = 6.93:1      | 97.8 mph | 87.6 mph @ 6000 rpm
       1    = 4.56:1      | 148.6 mph| 133 mph @ 6000 rpm

4.56 x 2 x 2.52 = 12.30:1 | 29.5     | 26.4 mph @ 6000 rpm
           1.52 = 13.86:1 | 48.9     | 43.8 mph @ 6000 rpm
           1    = 4.56:1  | 74.3     | 66.5 mph @ 6000 rpm

4.88 x 2.52 = 22.98:1     | 55.1 mph | 49.3 mph @ 6000 rpm
       1.52 = 7.42:1      | 91.3 mph | 81.8 mph @ 6000 rpm
       1    = 4.88:1      | 138.9 mph| 124.4 mph @ 6000 rpm

4.88 x 2 x 2.52 = 24.60:1 | 27.55    | 24.67 mph @ 6000 rpm
           1.52 = 14.84:1 | 45.6     | 40.9 mph @ 6000 rpm
           1    = 9.76:1  | 69.4     | 62.2 mph @ 6000 rpm
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38 inch tire with 4.56:1 ratio will do 60 mph @ 2500 rpm
34 inch tire with 4.56:1 ratio will do 60 mph @ 2700 rpm
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