How to Change the Gear Ratio With Tire Sizes

Gears ratios affect speed and torque in relationship to tire diameter.

Gear ratio is the relationship between the number of teeth on each gear in a gear assembly. For example, a simple two-gear system consist of a driver gear, which is the gear attached to the engine or whatever powers it, and the driven gear, which is the gear moved by the driver gear. The gear ratio is the number of teeth in the driven gear divided by the number of teeth in the driver gear. In general, a higher gear ratio means more horsepower or torque, but less fuel efficiency. When working with an axle gear ratio, one can change the "effective" gear ratio by adjusting the tire size.

Instructions

1. Determine your old axle gear ratio. If you do not know off hand, divide the number of teeth in the ring gear by the number of teeth in the pinion gear to get your axle gear ratio. In an axle, the "driver gear" is called the pinion gear and the "driven gear" is the ring gear. As an example, if you have 35 teeth in the ring gear and 10 teeth in the pinion gear, your old axle gear ratio is 3.5-to-1.

2. Determine your current tire diameter. If you do not know it, contact a tire shop or manufacturer and give them the make and model and they can tell you the diameter. As an example, let's assume it's 26 inches.

3. Decide the new axle gear ratio you want to reach. For example, let's assume you want to increase the horsepower or torque associated with your vehicle by increasing your axle gear ratio from 3.5-to-1 to 4.0-to-1.

4. Determine the new tire diameter required to reach the new axle gear ratio using the formula: New Tire Diameter = New Axle Gear Ratio/Old Axle Gear Ratio x Old Tire Diameter. Using the examples above, you need a tire diameter of 30 inches in diameter (4.0/3.5 x 26) to reach a gear ratio of 4.0-to-1.

5. Change all four of your car tires to the new tire diameter.

References

• 4LO.com: Tire Size Change
• Circle track: Rearend Gear Guide - Gear Ratio Rationale