About TireFit

Every tyre-size tool on the internet prints the same two columns: old diameter, new diameter, and a percentage between them. None of them answers the question the reader actually arrived with, which is not what are the numbers but should I do this.

So this site prints a verdict. It judges the change against the ±3% convention vehicle makers calibrate to, draws the tolerance band around the original tyre so you can see the new one land inside it or outside it, and then localises the one consequence that is genuinely regulated — the speedometer, under UN ECE R39, which is asymmetric in a way that catches people out and which nobody else in this niche mentions at all.

It is also honest about its own edges. Geometry owns rolling diameter completely and owns nothing else. Rim width, arch clearance, load rating and your car's own placard decide the rest, and the pages say so rather than implying a size string is a fitment guarantee.

Who builds it

TireFit is built and maintained by Amit Sharma, alongside a family of data sites that each take one public standard or dataset and do the arithmetic properly. It carries advertising and no sponsored placement: there is nothing on this site anybody can buy their way into, because there are no products on it to promote.

The method is the product

Everything here is computed, and the method page prints the formulas, the ±3% caveat, the indexing rule and the list of things this site refuses to guess at. If a figure is wrong it is wrong by arithmetic and can be demonstrated in a line — send it.

SOURCE·Metric size geometry + UN ECE R39 speedometer tolerance·REVIEWED ETRTO STANDARD