Tire Size Calculator — diameter & speedo error
Enter stock and new tire codes; see diameter difference and how far your speedo will read off. ±3% is the generally accepted safe limit.
Quick answer: Compare two tire specs (e.g. 205/55R16 vs 225/45R17): overall diameter, width, and the exact speedometer error percentage. Free, instant, in your browser — for safe plus-sizing.
How to use
- Enter your stock size and the candidate size.
General information only — not medical, financial or legal advice. Figures cross-checked against public official sources (IRS / HMRC / NHS / WHO guidance) as of 2026-08. For decisions that matter, confirm with a licensed professional in your country.
About this tool
A tire code 205/55R16 means 205 mm section width, sidewall = 55% of width, on a 16-inch rim. Overall diameter = rim + 2×sidewall. Changing diameter changes effective gearing and speedometer reading by the diameter ratio; the industry rule of thumb keeps the change within ±3% to protect ABS/accuracy. This tool does the full math for any two codes.