Gear Ratio & Speed Calculator
Calculate vehicle speed from RPM, gear ratio, final drive, and tire size — or find RPM at a given speed. Essential for gear selection and tire sizing.
Transmission gear ratio
Overall tire diameter (not rim size)
Leave at 1.0 for 2WD or high range
How Gear Ratios Determine Speed
Your vehicle speed at any RPM depends on three things: the transmission gear ratio, the final drive (axle) ratio, and the tire diameter. Together these determine how many times the engine turns for each rotation of the wheels.
The Formula
Speed (mph) = (RPM × Tire Diameter × π) / (Gear Ratio × Final Drive × 336)
The constant 336 converts from inches-per-minute to miles-per-hour.
Common Uses
- Tire sizing: What happens to my cruising RPM if I go from 26" to 28" tires?
- Gear selection: Which final drive ratio gives me 2,000 RPM at 70 mph in overdrive?
- Top speed: What speed does my engine hit at redline in top gear?
- Towing: What RPM am I turning at highway speed in 4th gear with my current setup?
Finding Tire Diameter
If your tire is in metric format (e.g. 275/60R15), you can calculate the overall diameter: sidewall height = width × aspect ratio / 25.4, then diameter = (sidewall × 2) + rim diameter. For 275/60R15: sidewall = 275 × 0.60 / 25.4 = 6.5", diameter = 6.5 × 2 + 15 = 28".