Final Drive Ratio Calculator
SI
Imp
Imperial
Inputs
View results
Target engine speed
(n_e)
Desired engine RPM at the selected road speed.
rpm
Road speed
(v)
Vehicle speed for the ratio check.
mph
Tire diameter
(D)
Rolling tire diameter. Measured rolling diameter is better than nominal sidewall math.
in
Transmission ratio
(G)
Selected gear ratio. Overdrive gears are less than 1.0.
Transfer ratio
(T)
Transfer-case or auxiliary ratio. Use 1.0 if none.
Results
Default result
Edit inputs
Final drive ratio
(F)
3.967
Pass
Also computed
Overall ratio
2.975
Wheel speed
(n_w)
840.3
rpm
Tire circumference
81.68
in
Method notes
2 notes
Assumes direct mechanical coupling with no converter slip, clutch slip or tire growth.
Use measured tire revs per mile when speedometer calibration or cruise RPM needs to be exact.
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