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.

–Stock diameter

–New diameter

–Speedo error

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); content last reviewed September 2026. 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.

FAQ

What's the safe limit?

±3% diameter change is the common guideline; beyond that, speedo, ABS and clearance need professional recalibration.

Does width matter?

Width affects grip and clearance, not speedo — the tool shows both but only diameter drives the error.

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The complete tire size calculator guide

Prefer the long version — a worked example, the mistakes people actually make, and answers to real questions? The blog guide Tire Size Calculator — Complete Guide, Mistakes & FAQ covers all of it step by step.

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Tire Size Calculator — diameter & speedo error

How to use

About this tool

FAQ

Rate this tool

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The complete tire size calculator guide