Tire Size Calculator

Reading a tire code

In 225/45R17, the 225 is section width in millimetres, the 45 is the aspect ratio — sidewall height as a percentage of width — and 17 is the wheel diameter in inches. So the sidewall is 225 × 0.45 = 101.25 mm, about 3.99 inches, and the overall diameter is 17 + 2 × 3.99 = 24.97 inches. Because the aspect ratio is a percentage, a wider tire with the same ratio is also taller.

The 3 per cent rule

Staying within about 3 per cent of the original overall diameter keeps speedometer error, ABS behaviour and transmission shift points close to what the manufacturer calibrated. Beyond that, the speedometer error becomes noticeable and clearance and load rating need checking too.

Why your speedometer reads low with bigger tires

The speedometer counts wheel rotations and multiplies by the circumference it was calibrated for. Fit a larger tire and each rotation covers more ground, so you are genuinely going faster than the display says. Fit a smaller one and the reverse happens. The odometer is wrong by the same proportion, which affects service intervals and any mileage-based record.

Diameter drives the speedometer, width drives the clearance

A speedometer counts wheel rotations and multiplies by an assumed rolling circumference set at the factory for the original tyre size. Fit a taller tyre and each rotation covers more ground, so the indicated speed reads low and the odometer under-records distance; a shorter tyre does the reverse. This is the reason the conventional guidance is to stay within about 3 per cent of the original overall diameter, which keeps the error small and, on many vehicles, keeps ABS and stability systems inside their calibrated assumptions.

Width and offset are a separate constraint with a more physical failure mode. A wider tyre needs clearance at full steering lock and at full suspension compression, and the point of contact is often the strut, the inner wheel arch liner or a suspension arm rather than anything visible when the car is parked. Load index and speed rating should meet or exceed the original specification, and a change in overall diameter can also require the TPMS and, on some vehicles, the speedometer calibration to be reset.

Frequently Asked Questions