Drive Train

Final Drive Ratio Computation

Final Drive Ratio Computation

At 07:35 PM 5/31/00 -0400, Rick Carter wrote:
>I know there is a formula to figure out the final
>drive knowing the speed (checked accurate by my GPS) 4th gear ratio 1:1 and
>tire circumference 80" but I need help.

The formula is:

Final ratio = (Tire Diameter in Inches x RPM) / (336 x MPH)

This is good for any gear, but the ratio is the final ratio, i.e., transmission x differential.
Tire Diameter must be in inches. The most accurate way of getting this is actually to measure the rolling circumference (mark the tire, roll the car one turn of the tire, measure the distance) then calculate the diameter. For the speeds that we are tediously discussing, tire growth and slippage can be ignored. The 336 is a constant that is derived from all of the other factors, such as pi. The formula can obviously be rearranged to solve for any factor if the other factors are known or assumed.

Wally Plumley
928 Specialists

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