Gear Ratio and Speed Calculator

The Gear Ratio and Speed Calculator tells you how fast a vehicle is traveling based on its drivetrain setup — useful for tuning gear ratios, evaluating axle swaps, or planning tire size changes. Enter your engine RPM, tire diameter, transmission gear ratio, and differential ratio, then select your preferred speed unit (MPH or KM/H). You'll get your vehicle speed as the primary result, plus total gear ratio, wheel RPM, and tire revolutions per mile as secondary outputs.

inches

First gear typically 3-4, overdrive less than 1

Common ratios: 3.08, 3.23, 3.73, 4.10

Results

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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Frequently Asked Questions

What is gear ratio and how does it affect vehicle performance?

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.

How do I find my vehicle's differential ratio?

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.

Why is tire diameter important in gear ratio calculations?

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.

What's the difference between transmission ratio and differential ratio?

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.

How does engine RPM relate to vehicle speed?

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.

What are common differential ratios for different applications?

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.

How do I calculate the total gear reduction?

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.

Can I use this calculator for motorcycles or other vehicles?

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.