Differential Gear Calculator

The Differential Gear Calculator finds the ideal gear ratio for your vehicle's differential based on your tire size and desired cruising performance. Enter your tire width, aspect ratio, and wheel size, then add your ring gear teeth, pinion gear teeth, target speed, and engine RPM to get your Differential Gear Ratio. Secondary outputs include tire diameter, tire circumference, revolutions per mile, and actual speed at your target RPM.

mm
%

Tire sidewall height as percentage of width

inches

Number of teeth on the ring gear

Number of teeth on the pinion gear

mph

Desired cruising speed

rpm

Target engine RPM at cruising speed

Results

Differential Gear Ratio

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Calculated Tire Diameter

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Tire Circumference

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Tire Revolutions per Mile

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Actual Speed at Target RPM

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Frequently Asked Questions

What is a differential gear ratio?

The differential gear ratio is the number of times the driveshaft rotates for each rotation of the wheels. It's calculated by dividing the ring gear teeth by the pinion gear teeth (e.g., 41÷10 = 4.10:1).

How does gear ratio affect vehicle performance?

Lower numerical ratios (3.08:1) provide better fuel economy and higher top speeds, while higher ratios (4.56:1) offer more torque multiplication for better acceleration and towing capability.

Should I change my gear ratio when installing larger tires?

Yes, larger tires effectively make your gear ratio numerically lower, reducing acceleration and torque. Installing a numerically higher gear ratio can restore performance lost from larger tires.

How do I find my current gear ratio?

You can find your gear ratio stamped on the differential cover, in your owner's manual, or by counting ring and pinion gear teeth. Some vehicles have a build sheet in the glove box with this information.

What's the difference between ring gear and pinion gear?

The ring gear is the large gear attached to the differential case, while the pinion gear is the smaller gear connected to the driveshaft. The pinion gear drives the ring gear to transfer power to the wheels.

Can I calculate the ideal RPM for highway driving?

Yes, this calculator shows your actual speed at a given RPM. For optimal fuel economy, most engines perform best between 1,800-2,200 RPM at highway speeds.

How accurate is the tire diameter calculation?

The calculation is very accurate for standard tire sizes. However, actual tire diameter can vary slightly between manufacturers and tire models, so measure your actual tire for the most precise results.

Will changing my gear ratio affect my speedometer?

If you change tire size, yes. The speedometer reads based on driveshaft rotations, so larger tires will make it read slower than actual speed. Some vehicles allow speedometer recalibration.