Enter your Engine RPM, Tire Diameter, Transmission Gear Ratio, and Differential (Rear Gear) Ratio into the Gear Ratio and Speed Calculator to find your Vehicle Speed, along with your Total Gear Ratio, Wheel RPM, and Tire Revolutions per Mile — choose your preferred Speed Unit to get results in mph or km/h.
Vehicle Speed
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Total Gear Ratio
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Tire Revolutions per Mile
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Wheel RPM
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Gear ratio is the relationship between the number of teeth on two gears. A higher numerical ratio provides more torque multiplication but lower top speed, while a lower ratio gives higher speed but less torque multiplication.
Check your vehicle's door jamb sticker, owner's manual, or VIN decoder. You can also physically count gear teeth or perform a wheel rotation test by marking the driveshaft and wheel, then rotating the wheel one full turn.
Tire diameter directly affects the final drive ratio. Larger tires effectively make your gears 'taller' (numerically lower), while smaller tires make them 'shorter' (numerically higher), affecting both acceleration and top speed.
Transmission ratio varies by gear selection (1st, 2nd, etc.) and provides the first stage of gear reduction. Differential ratio is fixed and provides the final gear reduction between the driveshaft and wheels.
Vehicle speed is determined by engine RPM divided by the total gear ratio (transmission × differential) and adjusted for tire circumference. Higher RPM or lower gear ratios result in higher speeds.
Economy cars typically use 2.9-3.4:1 for fuel efficiency. Performance cars often use 3.7-4.1:1 for acceleration. Trucks may use 4.1-5.3:1 ratios for towing capability.
Multiply the transmission gear ratio by the differential ratio. For example, 1st gear (3.27:1) × differential (3.73:1) = 12.2:1 total reduction, meaning the engine turns 12.2 times for each wheel revolution.
Yes, the principles are the same for any vehicle with geared transmission. Just ensure you're using the correct gear ratios and tire dimensions for your specific vehicle type.