TreadIndex is an independent site that converts and compares tire sizes, and works out what changes when you fit a different one.
It exists because a tyre size is three numbers in two different unit systems, and every question people actually ask — will it rub, does the speedometer lie, what happens to my gearing — needs those three converted first.
Where the numbers come from
- the ISO metric tire size system and the flotation sizing convention
- the ±3% overall diameter window the industry works to
- published axle ratios actually manufactured, rather than the exact arithmetic answer
Every page states the source and the assumptions behind its figures, because a number without its assumptions is not much use. Where a clearance figure is a rule of thumb rather than a measurement, the page says so.
Clearance gains half the diameter
Ground clearance changes by half the diameter change, not all of it — the tire grows at the top and the bottom, and only the top half lifts the axle. It is the point most lift-and-tire discussions get muddled.
What this site does not do
We do not advise on load index or speed rating substitutions. Fitting a tire rated below the vehicle placard is a safety decision, not an arithmetic one.
Who runs it
One person who has fitted the wrong size at least once and would rather you did not. There is no editorial team. What should earn your trust instead is that every conversion here is arithmetic you can repeat, with the formula shown.
The conversions run as code across every size pair the site offers, so a mistake in one constant would be wrong across thousands of comparisons and get spotted. Errors are corrected and the correction is recorded on the page.
How it is paid for
TreadIndex is free to read and funded by display advertising. Nothing here is sponsored and there are no affiliate links; no tyre brand has any say in what these comparisons show.
Getting in touch
Corrections and questions go to [email protected]. If a figure is wrong, send it — the editorial policy covers what happens to corrections.