How much size difference is safe?

Keep overall diameter within about 3 per cent of the original. Beyond that, speedometer and odometer error grows and electronic systems calibrated to the original rolling radius can behave unexpectedly.

What does the R in 225/45R17 mean?

Radial construction, which is standard on virtually every modern passenger car. It describes how the internal plies are laid, not a dimension.

Does a wider tire always grip better?

Not automatically. A wider tire adds contact area but also weight, rolling resistance and tramlining, and it can reduce wet performance if the tread cannot clear water across the extra width.

Tire Size Calculator

Quick answer

Overall tire diameter equals the rim diameter in millimetres plus twice the sidewall height, where sidewall height is section width × aspect ratio ÷ 100. A 225/45R17 measures about 634 mm.

Tire dimensions from the code

Enter the three numbers from a code like 225/45R17.

Section width (mm)

Aspect ratio (%)

Rim diameter (inches)

Overall diameter

634.3 mm

Sidewall height

101.3 mm

Revolutions per km

502

Compare this diameter with your original size. Keep the difference under about 3% to avoid speedometer and ABS calibration problems.

A tire code like 225/45R17 packs three different units into one string. The 225 is section width in millimetres, the 45 is the aspect ratio — sidewall height as a percentage of that width — and the 17 is rim diameter in inches. Mixing millimetres and inches in one code is why the arithmetic trips people up.

The number that matters when changing size is overall diameter. Change it and you change your effective gearing, your speedometer reading and, on many cars, the calibration that traction control and ABS rely on.

Why the speedometer drifts after a size change

Speed is derived from wheel rotation, so a larger-diameter tire travels further per revolution and the car reads slower than it is actually going. A 2 per cent larger diameter means the speedometer under-reads by about 2 per cent — showing 98 when you are doing 100.

Most manufacturers and regulators treat a change of up to roughly 3 per cent as acceptable. Beyond that you risk odometer error, incorrect cruise control behaviour and, in some jurisdictions, a failed inspection.

Plus sizing: keeping diameter while changing the rim

Going up an inch in rim size while keeping overall diameter roughly constant means widening the tire and dropping the aspect ratio. From 205/55R16 you would move to about 225/45R17, which keeps diameter within a per cent.

The trade is ride comfort and rim protection for steering response. A shorter sidewall transmits more road surface to the cabin and leaves less rubber between a pothole and an expensive alloy.

Frequently asked questions

How much size difference is safe?: Keep overall diameter within about 3 per cent of the original. Beyond that, speedometer and odometer error grows and electronic systems calibrated to the original rolling radius can behave unexpectedly.
What does the R in 225/45R17 mean?: Radial construction, which is standard on virtually every modern passenger car. It describes how the internal plies are laid, not a dimension.
Does a wider tire always grip better?: Not automatically. A wider tire adds contact area but also weight, rolling resistance and tramlining, and it can reduce wet performance if the tread cannot clear water across the extra width.

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Updated 2026-09-20 · all calculators. Results are estimates for guidance, not professional advice.