About the tool
Clear tire math, fewer surprises.
FitMyTires helps drivers understand the dimensional effects of a proposed tire and wheel change before they spend money or schedule installation.
What it can tell you
Using nominal tire and wheel dimensions, the calculator shows changes in overall diameter, sidewall, circumference, revolutions per mile, indicated speed, ride height, and estimated inner and outer position. Results are shown in inches or millimeters.
What it cannot tell you
It does not know your vehicle's measured fender, suspension, brake, hub, fastener, load, or steering clearances. It cannot approve a fitment or replace a physical test-fit and professional inspection.
How the numbers are calculated
Every page uses the same calculation code. For a metric size such as 225/45R17, sidewall height is the section width × aspect ratio ÷ 100, and overall diameter is two sidewalls plus the rim diameter. Values keep full precision and are rounded only for display.
If you enter a tire model's published diameter or revolutions per mile, the calculator uses it and labels it as user supplied and unverified. Size and comparison pages cover only sizes with a documented source: historical NHTSA complaint records or current Michelin size listings. Read the full methodology.
Accounts, ads, and analytics
No account and no ads. We use Google Analytics and Cloudflare Web Analytics to see which pages help. Exact calculator inputs are not sent to Google Analytics. Read the privacy policy.
Publisher and review
FitMyTires publishes and maintains the calculator, its source-backed size and comparison pages, and its supporting policies. The math is covered by automated tests. The site and its core methodology were last reviewed September 29, 2026.
Report an error
If a number, source, or explanation looks wrong, email hello@fitmytires.com with the page address or the sizes you compared and what you expected. Confirmed errors are fixed in the shared calculation or source data, so every affected page updates.
Read the editorial and corrections policy, or inspect the formulas and source criteria.
Last updated: .