Speedo error calculator calculates speedometer error as a result of the change in tyre size on a vehicle.

It shows the actual speed your car is going after the tyre size change - installing the new tyres with a larger or smaller tyre diameter than the original equipment(OE) tyre. Also it might help with the speedometer calibration, to determine how much you need to calibrate your speedometer when you upgrade/downgrade your wheel/tyre size.

To find out speedometer error, in the calculator below select the original tyre size, the new tyre size, enter kilometers or miles per hour into the "Enter Speedometer Reading" section and then click on the "Calculate Actual Speed" button to see how much faster or slower your car actually is going at different speed readings on your speedometer .

At the bottom of the page read more about speedometer error. Also, you can see an animation which shows why the speedo error happens.

Speedo Error Calculator

select original tyre size

eg. 195/65R15

/R
select new tyre size
/R
KMH or MPH

Speedometer reads , but the actual speed is km/h / mph.

Disclaimer:
This calculator shows the user a general idea of the wheel/tyre changing impact on the vehicle speed. The calculator doesn't take into account curvature effects of a tyre when placed on a rim it is not designed for or any other side effects (tyre pressure, tyre worn out,...). The tyresizecalculator.com is not responsible for any miscalculations of this calculator or any errors as a result of curvature effects or any other side effects.

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## Speedometer error

Speedometer (Speedo) is an instrument that measures the speed at which the vehicle is moving at a given moment. It usually includes a distance traveled recording mechanism (odometer).

Speedometer Error could arise from many reasons, and one of them is the variation of tyre diameter as a result of changing tyre size.

Eg. If you install tyres on your vehicle, with a diameter larger than your original equipment(OE) tyres have, the wheel with the new, bigger tyre, with each revolution, will cover greater distance as compared to the original tyre, for the same time interval (see picture below). Because speed is defined as the distance covered per unit of time, your actual speed will be greater than the speedo reading. A speedometer will read the same speed as it is with the original tyres. This is because of the inner workings of the speedometer calibration. A car speedometer doesn't actually measure how fast you travel from point to point, it usually work by measuring rotation of the car's driveshaft, axle or wheel.

Because of that variation in the actual speed the need for the speedometer calibration might arise. This calculator also could help you determine how much you need to calibrate your speedometer when you upgrade wheel size/tyre.

That is why it's important that, when you change tyre size on your vehicle, try to persist approximately the same overall tyre diameter as the one the original equipment(OE) tyre has – method called Plus Sizing. Our Plus Sizing Calculator could help you to choose the optimal tyre size.

Animation shows the one revolution of a wheel with different tyre sizes and the tyre size impact on the car actual speed.