Enter your original tire specs (Section Width, Aspect Ratio, Rim Diameter) and your new tire specs, then add your current Speedometer Reading to find out your Actual Speed — along with the Speed Error, Error Percentage, and both tire diameters so you know exactly how far off your gauge really is.
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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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.
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.
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.
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.
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.
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.
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.
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.