Overall Gear Ratio with Tire Size Calculator

When you swap to larger or smaller tires, your vehicle's effective gear ratio changes — meaning your engine works harder or easier than your axle gearing was designed for. The Overall Gear Ratio with Tire Size Calculator lets you enter your stock tire size (width, aspect ratio, rim diameter), new tire size, and stock gear ratio to find your effective gear ratio after the tire change. Secondary outputs include the new gear ratio needed to restore original performance, both tire diameters, and the percentage diameter change.

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Your current differential gear ratio

Results

Effective Gear Ratio

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New Gear Ratio Needed

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Stock Tire Diameter

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New Tire Diameter

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Diameter Change

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Frequently Asked Questions

How does tire size affect gear ratio?

Larger tires effectively lower your gear ratio, reducing acceleration but improving top speed and fuel economy. Smaller tires have the opposite effect, creating a higher effective gear ratio with better acceleration but lower top speed.

What is an effective gear ratio?

The effective gear ratio is your actual final drive ratio after accounting for tire size changes. It's calculated by multiplying your differential gear ratio by the ratio of stock tire diameter to new tire diameter.

Do I need to change my differential gears when upgrading tire size?

Not necessarily, but changing to a numerically higher gear ratio can restore your original performance characteristics. This is especially important with significantly larger tires to maintain acceleration and reduce stress on the drivetrain.

How do I calculate tire diameter from tire size?

Tire diameter = (2 × sidewall height) + rim diameter. Sidewall height = (tire width × aspect ratio) / 25.4. For example, a 235/70R16 tire has a diameter of approximately 28.0 inches.

What gear ratio should I choose for larger tires?

Use the 'New Gear Ratio Needed' result to maintain stock performance. For better acceleration with larger tires, choose a slightly higher numerical ratio. For highway driving, you might prefer keeping the lower effective ratio.

Will changing gear ratios affect my speedometer?

No, gear ratio changes don't affect speedometer accuracy. However, the tire size change that prompted the gear ratio change will affect speedometer readings. Larger tires make the speedometer read slower than actual speed.

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

Ring and pinion gear ratio refers to your differential's final drive ratio - the ratio between the ring gear teeth and pinion gear teeth. Common ratios include 3.08, 3.31, 3.55, 3.73, 4.10, and 4.56.