Performance & Engine

Gear Ratio Speed

Calculate road speed from gear ratios and tire size.

How to Calculate Gearing and Speed

V = (RPM × C) / (GR × FD × 1056)

Where: V = vehicle speed in mph RPM = engine revolutions per minute C = tire circumference in inches GR = transmission gear ratio FD = final drive ratio 1056 = conversion constant

Engine RPM

Tire Circumference (inches)

Gear Ratio (e.g., 3.42)

Final Drive Ratio (e.g., 3.73)

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About this tool

The BuildLockr gear ratio & speed calculator translates engine speeds into true road velocities. Use this tool to instantly calculate vehicle speed in MPH and KM/H based on your engine RPM, transmission gears, final drive differential ratio, and tire circumference. Whether you are modeling custom gearbox ratios or figuring out highway RPMs with oversized tires, this gearing speed calculator delivers precise estimates. This helps drag racers and track builders select the ideal final drive gears for their target speeds.

Understanding Gear Ratios

Gear ratios determine how engine RPM translates to wheel speed. Lower (numerically higher) gear ratios provide more torque multiplication but lower top speed, while higher (numerically lower) ratios allow higher speeds but less acceleration.

Benefits

• Helps optimize gearing for specific track conditions
• Allows calculation of optimal shift points
• Useful for predicting top speed in each gear
• Essential for transmission and differential setup

Key Considerations

• Shorter gearing (higher numerical ratio) improves acceleration
• Taller gearing (lower numerical ratio) improves fuel economy
• Final drive ratio affects all gears equally
• Tire size changes effectively alter your overall gearing

Common Gearing Questions

How do you calculate vehicle speed from RPM and gear ratio?

Vehicle speed is calculated using engine RPM, gear ratio, final drive ratio, and tire size. The formula is: Speed (MPH) = (RPM × Tire Circumference in inches) / (Gear Ratio × Final Drive Ratio × 1056).

What is the difference between a transmission ratio and a final drive ratio?

The transmission gear ratio determines the speed difference between the engine crankshaft and the transmission output shaft for a specific gear. The final drive ratio (found in the differential or transaxle) multiplies the torque and reduces the speed of the output shaft before it reaches the wheels, affecting all gears equally.

How does tire size affect overall gearing?

Taller tires have a larger circumference, meaning they travel further per revolution. This effectively lowers the numerical gear ratio (tall gearing), increasing top speed in each gear but reducing torque multiplication and acceleration force.