How to Figure Differential Gear Ratio

Lower gear ratios deliver more torque to the tires.

Differential gear ratio is the number of times the drive shaft turns each time a vehicle's tires make one rotation. Different ratios are ideal in different situations: lower ratios are better for off-road use, while higher ratios provide better fuel economy and are more desirable for highway driving. The easiest way to determine the ratio is to divide the number of teeth on the ring gear by the number of teeth on the pinion. However, the ratio can also be determined by spinning the tires and counting the rotations. The process varies depending on whether or not the vehicle has positraction.

Things You'll Need

• Car jack
• Two 1-inch pieces of duct tape

Instructions

Vehicles without Positraction

1. Place the jack under the car near one of the drive wheels. Jack the car up until the tire is off the ground.

2. Place one piece of tape at the 12 o'clock position on the tire and one piece of tape on the drive shaft facing you. This will mark the starting positions.

3. Place the car in neutral, and then spin the tire 10 complete revolutions while counting the number of times the drive shaft spins.

4. Divide the number of revolutions of the drive shaft by five, or half the number of revolutions of the tire. For example, if the drive shaft spins 18 times over the 10 revolutions of the tire, the differential gear ratio would be 18/5 = 3.6.

Vehicles with Positraction

1. Place the jack under the car near the drive wheels and jack up both drive wheels.

2. Place one piece of tape at the 12 o'clock position on one of the tires and one piece of tape on the drive shaft facing you. This will mark the starting positions.

3. Place the car in neutral, and then spin the tire 10 complete revolutions while counting the number of times the drive shaft spins.

4. Divide the number of revolutions of the drive shaft by the number of revolutions of the tire. For example, if the drive shaft spins 36 times over the 10 revolutions of the tire, the differential gear ratio would be 18/10 = 3.6.

Tips & Warnings

• Check the vehicle's user manual to see whether it is rear-wheel or front-wheel drive.

• Use caution when working under a vehicle on a jack. Do not attempt to lift a vehicle while on an incline.

References

• Frequently Asked Questions about Differentials
• Computer Support Group Network: Ring Gear--Pinion Gear Calculator