# Definition of a Final Drive Ratio

by Jennifer VanBaren

The final drive ratio is used in the car industry and tells how many rotations your motor must make for each rotation of the wheels. This ratio is among several ratios used for comparing and determining the performance in vehicles. It is not hard to calculate if you have the correct information.

### Purpose

Ratios are used in virtually every industry for various purposes, including the automobile industry. The final drive ratio, among other car ratios, is calculated and used to determine the quality of a car's performance. This ratio can help you determine the correct tire diameter and gear ratios needed to optimize the performance of your vehicle.

### Formula

The final drive ratio is calculated by multiplying the primary gear ratio (P) times the internal transmission ratio (I). This ratio takes into account every single gear in between the motor and the wheels. The answer to the ratio varies based on the type of car, the engine, the wheels, the number of teeth on each pinion gear and many other factors.

### Information

If you know the primary gear ratio and the transmission ratio, you can find the answer by multiplying these two items. Otherwise to determine this ratio, call a car dealership or service department and find out how many teeth are on the spur and pinion gears of your car. Locate the owner's manual of your vehicle to find out what your car's transmission drive ratio is; which is normally found under the section titled "Vehicle Specifications." The primary gear ratio is determined by dividing the number of spur gear teeth by the number of pinion gear teeth. Complete the ratio by multiplying this number times the transmission ratio.

### Details

After calculating the final drive ratio, it is written in terms of a ratio such as 2.5:1. This means that the pinion must turn 2.5 times in order to turn the wheels or drive axle once. The higher the number is, the shorter the gear is. Short gears offer quick acceleration, but also have negative effects including lower gas mileage and lower top speeds.