Tire Size Calculator

How to use the tire size calculator

1. Fill in the fields — they start with a worked example.
2. Read the answer, and the arithmetic shown beneath it.
3. Copy the results, or adjust the inputs to compare scenarios.

About this tool

A tire size such as 225/45R17 is three separate measurements. 225 is the tread width in millimetres, 45 is the sidewall height as a percentage of that width, and 17 is the wheel diameter in inches. Mixing millimetres, a ratio and inches in one code is confusing, and it is why the effect of a size change is hard to judge by eye.

The number that matters is overall diameter, because the speedometer is driven from wheel rotation. Fit a taller tire and each revolution covers more ground, so the car is travelling faster than the dial says; fit a shorter one and the dial reads high. A change of more than about three per cent is usually considered too much, and in many places the odometer error that comes with it has legal and warranty consequences.

Sidewall height is the other practical figure. Going up a wheel size while keeping the diameter constant — plus-sizing — means a shorter sidewall, which sharpens the steering and makes the ride firmer and the wheel easier to damage on a pothole. The calculator shows both sidewalls so the trade is explicit.

Diameter also affects gearing and, on many cars, the ABS and stability systems, which infer wheel speed from the same rotation count. That is the argument for staying close to the original size rather than merely close enough.

Common uses

• Checking how far a replacement tire size shifts the speedometer.
• Comparing a plus-size wheel and tire package against the original.
• Working out the sidewall height lost when moving up a wheel size.

Frequently asked questions

What do the numbers in 225/45R17 mean?

225 is the tread width in millimetres, 45 is the sidewall height as a percentage of that width, and 17 is the wheel diameter in inches. R means radial construction.

How much tire size difference is acceptable?

Within about three per cent of the original overall diameter is the usual guidance. Beyond that the speedometer, odometer and wheel-speed-based systems all drift measurably.

Will bigger tires make my speedometer read low?

Yes. A taller tire covers more ground per revolution, so you are actually going faster than the dial shows, and the odometer records fewer miles than you drove.