Tyre Size Calculator
The middle number is a percentage, not a measurement.
Work out Tyre Size. The middle number is a percentage, not a measurement. Shows the working, not just the answer.
Overall diameter
634.3 mm
-3.43% against the original size
The middle number in a tyre size is a percentage, not a measurement. 225/45R17 is 225 mm wide with a sidewall 45% of that — 101 mm — on a 17-inch rim, giving 634 mm overall. Because the sidewall is a ratio, a wider tyre in the same size code is also a taller one. Changing overall diameter changes what the speedometer reads, since it is calibrated for the original size. A taller tyre makes the needle under-read, so you are going faster than it says — which matters more than the reverse. Most jurisdictions accept within about 3% of the plated diameter, and manufacturers usually design their own alternative fitments inside 1%. Beyond that you affect the speedometer, the odometer, ABS calibration and, on some cars, the stability control.
How the Tyre Size Calculator works
Overall diameter from a tyre size code, and the speedometer error against your original size. 225/45R17 is 225 mm wide with a sidewall 45% of that — a ratio, which is why a wider tyre in the same code is also a taller one.
Also known as: will bigger tyres affect my speedo · what does 225 45 r17 mean · tyre size comparison calculator · can i fit different size tyres
Frequently asked questions
What do tyre size numbers mean?
Width in millimetres, sidewall as a percentage of that width, then rim diameter in inches. 225/45R17 is 225 mm wide, a 101 mm sidewall and a 17-inch rim, giving 634 mm overall.
Will bigger tyres affect my speedometer?
Yes. The speedometer is calibrated to the original diameter, so a taller tyre makes it under-read — you are going faster than it shows. A shorter tyre does the reverse, which is the safer error of the two.
How much can I change tyre size legally?
Most jurisdictions accept within about 3% of the plated diameter, and manufacturers keep their own alternative fitments inside 1%. Beyond that you affect the speedometer, odometer, ABS calibration and on some cars the stability control.
What is plus sizing?
Fitting a larger rim with a lower-profile tyre to keep the overall diameter the same. It improves steering response and looks, at the cost of ride comfort and a much higher risk of pothole damage to the wheel.
Does a wider tyre have more grip?
Usually somewhat, in the dry, at the cost of fuel economy, aquaplaning resistance and steering weight. The compound and construction matter more than the width, which is why a good narrow tyre outperforms a cheap wide one.
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