Engine RPM to Vehicle Speed: Sizing Tires & Gear Ratios

To determine how fast a vehicle travels at a specific engine RPM, you must calculate how many times the tires rotate based on the transmission ratio, final drive ratio, and tire diameter.

The Speed Formula

Vehicle speed in miles per hour (mph) is calculated using this formula:

$$\text{Speed (mph)} = \frac{\text{RPM} \times \text{Tire Diameter (inches)}}{\text{Gear Ratio} \times \text{Final Drive} \times 336}$$

For example, an engine spinning at 3,000 RPM with a 1.00 gear ratio, 3.73 final drive, and 26-inch tires is traveling at approximately 62 mph ((3000 \times 26 \div (1.00 \times 3.73 \times 336) = 62.2\)).

Running speed simulation profiles on the automotive gear ratio calculator helps developers choose between performance-oriented close vs wide gearboxes.