Transmission Calculator
Find vehicle speed from engine rpm, gearing and tire size.
Find road speed for a given engine rpm, gear, final drive and tire size.
How the Math Works
The formula speed = rpm·π·tire·60 ÷ (gear·final·63360) calculates vehicle speed by relating engine revolutions to wheel rotation. Engine RPM measures how many times the crankshaft spins per minute, while the tire diameter determines the distance traveled per revolution (circumference = π·tire). Multiplying RPM by tire circumference gives inches per minute, and multiplying by 60 converts this to inches per hour. The denominator accounts for gear ratios (gear and final) that reduce engine RPM before reaching the wheels, and 63360 converts inches to miles (since 1 mile = 63,360 inches). The result is speed in miles per hour.
Practical Applications
To use this calculator, input the engine RPM, tire diameter in inches, and the combined gear ratio (transmission gear × final drive ratio). For example, if your engine is at 3,000 RPM with 32-inch tires and a 3.73 final drive ratio in 4th gear (1.5:1), the calculation would be: (3000 × π × 32 × 60) ÷ (1.5 × 3.73 × 63360) ≈ 55 mph. This helps when adjusting gear ratios, swapping tires, or verifying speedometer accuracy after modifications.
Day-to-Day Use
This calculation is essential for diagnosing speedometer discrepancies, optimizing gear selection for towing or racing, and ensuring accurate fuel economy tracking. If you install larger tires, your speedometer may read slower than your actual speed—this formula corrects that. It also helps prioritize maintenance (e.g., checking tire wear patterns) and planning trips by predicting fuel usage based on RPM and gear settings.
Worked example
3000 rpm, 1:1 gear, 3.7 final, 26" tire → about 65 mph.
FAQ
Why do taller tires raise speed?
They travel farther per revolution, lowering rpm at a given speed.