Tire Size Calculator
Compare original and new tire sizes. Calculate diameter, circumference, percentage difference and theoretical speed at your indicated speed.
Jump to 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.
Compare original and new tire sizes
- Original diameter
- 631.9 mm
- New diameter
- 634.3 mm
- Original circumference
- 1,985.17 mm
- New circumference
- 1,992.71 mm
- Diameter difference
- 0.38 %
- Theoretical actual speed
- 60.23 same unit as input
Speed assumes the original tire calibration. This geometry comparison does not confirm clearance, load rating, legal compliance or vehicle compatibility. Check the vehicle manufacturer specifications.
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.
A diameter percentage alone does not establish safe fitment. Clearance, load and speed ratings, vehicle calibration and local requirements must be checked against manufacturer specifications.
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?
- There is no universal safe percentage. This tool compares geometry only; verify approved sizes, clearance, load and speed ratings with the vehicle manufacturer.
- 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 2026-10-06 · all tools. Results are estimates for guidance, not professional advice.