How it works
Each turn of the tire moves the vehicle one circumference (π × diameter). The axle ratio and the transmission gear multiply how many times the engine turns for each tire revolution.
Tire revolutions per mile = 63,360 ÷ (π × diameter in inches)
RPM = mph × axle ratio × gear ratio × 63,360 ÷ (60 × π × diameter)
RPM ≈ mph × axle ratio × gear ratio × 336.14 ÷ diameter
Worked example
60 mph in direct drive (1.00), a 3.73 axle and 32 in tires.
- RPM: 60 × 3.73 × 1.00 × 336.13 ÷ 32 ≈ 2,351 RPM
- In a 0.67 overdrive: 2,351 × 0.67 ≈ 1,575 RPM
- Tire revolutions per mile: 63,360 ÷ (π × 32) ≈ 630
RPM at 60 mph in a 1:1 gear
| Axle ratio | 30 in tires | 32 in tires | 33 in tires | 35 in tires |
|---|---|---|---|---|
| 3.21 | 2,158 | 2,023 | 1,962 | 1,850 |
| 3.42 | 2,299 | 2,155 | 2,090 | 1,971 |
| 3.73 | 2,508 | 2,351 | 2,280 | 2,149 |
| 4.10 | 2,756 | 2,584 | 2,506 | 2,363 |
| 4.30 | 2,891 | 2,710 | 2,628 | 2,478 |
| 4.88 | 3,281 | 3,076 | 2,982 | 2,812 |
Multiply by your top gear’s ratio for cruising RPM. For example, a 0.70 overdrive gives 70% of the number in the table.
Why RPM matters when towing
An engine makes its pulling power over a range of RPM. Too low and it lugs, struggling and running hot; too high and fuel use climbs. Towing usually works best a gear or two below top gear, which is why many trucks have a tow/haul mode that holds lower gears. A numerically higher axle ratio (4.10 instead of 3.21) raises RPM at any speed and multiplies torque at the wheels, which helps heavy towing at the cost of fuel economy.
Bigger tires do the opposite and lower RPM; the effective gear ratio calculator shows how much. Find a tire’s diameter with the tire size calculator, and see the power needed on hills with the towing horsepower calculator.
Common mistakes
- Using the rim size as the diameter. Use the full tire diameter, sidewalls included.
- Forgetting the gear. The result is for the gear ratio you enter; overdrive changes it a lot.
- Trusting the speedometer after a tire change. Taller tires make the speedometer read low.
- Mixing up the ratio direction. Axle ratios are written as 3.73:1; enter 3.73.