IMPERIAL
The overall gear reduction is the RPM time the rolling radius of the loaded driving tyre divided by 168 times the vehicle speed in miles per hour
![1](https://fr.webtec.com/api/i.aspx?wteqi11a.gif;82;29)
Where:
168 = Factor
RPM = Revolutions per minute of engine
r = Rolling radius of loaded driving tyre in inches
R = Overall gear reduction including both axle and transmission
MPH = Vehicle speed in miles per hour
Example:
Find out overall gear reduction of a vehicle where the motor speed is 1680 RPM, the rolling radius of loaded driving tyre is 15 inches and the MPH is 15.
![2](https://fr.webtec.com/api/i.aspx?wteqi11b.gif;117;29)
METRIC
The overall gear reduction is the RPM time the rolling radius of the loaded driving tyre divided by 168 times the vehicle speed in miles per hour
![3](https://fr.webtec.com/api/i.aspx?wteqm11a.gif;101;29)
Where:
2651.51 = Factor
RPM = Revolutions per minute of engine
r = Rolling radius of loaded driving tyre in millimetres
R = Overall gear reduction including both axle and transmission
KPH = Vehicle speed in kilometres per hour
Example:
Find out overall gear reduction of a vehicle where the motor speed is 1680 RPM, the rolling radius of loaded driving tyre is 381mm and the KPH is 24.
![4](https://fr.webtec.com/api/i.aspx?wteqm11b.gif;133;29)