How to Calculate Gear Ratio to Tire Size

A fixed engine speed and gear ratio need larger tires to achieve a higher vehicle speed.

Gear ratio relates an engine's speed with its axles' speed, measured in revolutions per minute. Drivers easily determine their engine speed by checking their dashboard's relevant dial. No dial reveals the axle's speed, but you can calculate it from the vehicle's linear speed and the tires' size. If you know the gear ratio but not the tire size, use the engine speed, vehicle speed and gear ratio to calculate the size of the tires.

Instructions

1. Multiply the gear ratio by the engine speed in revolutions per minute. If the gear ratio is 5.35 and the car runs at 4,000 RPM: 5.35 x 4,000 RPM = 746.97. This is the axle speed in revolutions per minute.

2. Multiply the answer by 2.

   746.97 x 2 = 1,493.94

3. Multiply the answer by pi.

   1,493.94 x 3.14 = 4,691

4. Divide by the vehicle's speed in miles per hour. If the car travels at 40 miles per hour: 4,691/40 = 117.3.

5. Divide 1,056 by your answer: 1,056 /117.3 = 9. This is the tire's radius in inches.

References

• "Auto Math Handbook: Calculations, Formulas, Equations and Theory"; John Lawlor; 1991
• Shelquist Engineering: Speed Versus RPM Calculator