Speedometer Error Calculator

When you swap to a different tire size, your speedometer no longer reads correctly — because it was calibrated for your original tires. Enter your original tire size (section width, aspect ratio, rim diameter) and your new tire size into the Speedometer Error Calculator, along with your speedometer reading and speed unit, to find your actual speed. Secondary outputs include the speed error, error percentage, and both tire diameters for comparison.

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Results

Actual Speed

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Speed Error

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Error Percentage

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Original Tire Diameter

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New Tire Diameter

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Frequently Asked Questions

How does changing tire size affect speedometer accuracy?

Speedometers measure wheel rotations, not actual speed. Larger tires travel further with each rotation, so your actual speed is higher than the speedometer shows. Smaller tires have the opposite effect.

Will bigger tires make my speedometer read slower?

Yes, with bigger tires your actual speed will be higher than your speedometer reading. The speedometer will show a lower speed than you're actually traveling.

How do I calculate tire diameter from tire size?

Tire diameter equals rim diameter plus twice the sidewall height. Sidewall height is calculated as (section width × aspect ratio ÷ 100). Convert millimeters to inches by dividing by 25.4.

Can I get a ticket for speedometer error?

Yes, law enforcement measures your actual speed, not your speedometer reading. You're responsible for knowing your true speed regardless of speedometer error from tire changes.

How much speedometer error is acceptable?

Most automotive experts recommend keeping speedometer error within 3% to avoid significant inaccuracy. Errors above 5% can noticeably affect speed judgment and fuel economy calculations.

Does speedometer error affect my odometer?

Yes, odometer readings will also be affected by tire size changes. Larger tires will cause the odometer to under-report actual distance traveled, while smaller tires cause over-reporting.

Can I recalibrate my speedometer after changing tire size?

Modern electronic speedometers can sometimes be recalibrated through the vehicle's computer system. Older mechanical speedometers may require gear changes in the transmission or differential.

What's the difference between plus sizing and changing overall diameter?

Plus sizing maintains the same overall tire diameter while using a larger rim and shorter sidewall. This preserves speedometer accuracy while changing the tire's performance characteristics.