About tiresizecalculator.app
What this site is
tiresizecalculator.app is a free set of tire size calculators: compare sizes with a drawing to scale, see speedometer error and engine speed, convert metric and inch sizes, decode the sidewall and find plus sizes. It is published by Takreparation Skåne AB in Ängelholm, Sweden.
How it calculates
| Value | Formula | Example: 265/70R17 |
|---|---|---|
| Sidewall height | width × aspect ratio ÷ 100 | 265 × 70 ÷ 100 = 185.5 mm (7.30 in) |
| Overall diameter | rim × 25.4 + 2 × sidewall | 17 × 25.4 + 2 × 185.5 = 802.8 mm (31.6 in) |
| Circumference | π × diameter | 3.1416 × 802.8 = 2522 mm (99.3 in) |
| Revolutions per mile | 63,360 ÷ circumference in inches | 63,360 ÷ 99.3 = 638 |
| Actual speed | speedometer × new diameter ÷ old diameter | 60 × 830.8 ÷ 802.8 = 62.1 mph on 285/70R17 |
These are nominal dimensions from the size code. Real tires differ slightly by make, model, tread depth and the rim they are fitted to.
Load index and speed symbol values are the standard ETRTO values as listed in Wikipedia: Tire code. The speedometer rule quoted is from UN Regulation No. 39, paragraph 5.4 (EUR-Lex).
What we do not claim
The site calculates geometry from the size code. It does not know your vehicle, wheels or local rules and does not approve any size. There are no experts or certificates behind this site; the formulas are shown openly. Always check your vehicle owner’s manual, the tire placard in the door jamb, or a tire shop before changing size.
No account, no tracking
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Last updated: October 15, 2026