How We Calculate — Method & Data Notes

Every number on this site comes from one formula set, computed the same way on every page. Here is exactly what we do, and what the numbers are not.

The formulas

For a metric size like 265/70R17: width = 265 mm; sidewall = width × aspect = 265 × 70% = 185.5 mm; diameter = rim (17 × 25.4 = 431.8 mm) + 2 × sidewall = 802.8 mm = 31.61". Circumference = π × diameter; revolutions per mile = 63,360 ÷ circumference in inches. For a flotation size like 33x12.50R17, the first number is the nominal diameter in inches and the second the width in inches. The width figure is the nominal section width — sidewall to sidewall. The tread itself is narrower and varies by model, so we never label the section width as tread width.

Speedometer effect: actual speed = indicated speed × (new diameter ÷ old diameter), assuming the speedometer was accurate on the old tire.

Nominal vs measured

Size codes are nominal. Two tires stamped 285/70R17 from different makers can measure a few tenths of an inch apart, and tread wear and load change the rolling height. We publish the nominal geometry — the neutral reference sizes are compared on — and say so on every page. For one exact tire model, the maker's own spec sheet (which lists measured diameter and approved rim widths) is the source to trust.

Why your actual tire may measure differently

Five things move a real tire off its nominal numbers: 1) the maker and model — casings and tread blocks differ; 2) the measuring rim width — the same tire sits wider or narrower on a different wheel; 3) tread wear — a worn tire is measurably shorter; 4) inflation pressure — under-inflation squashes the loaded radius; 5) load — weight flattens the contact patch. That is also why the revolutions-per-mile figures here are labeled nominal: they come straight from nominal circumference, while a maker's published figure is measured on a rolling tire under load.

Load index & speed ratings

Load and speed figures follow the standard published scales used across the industry (for example load index 115 = 1,215 kg = 2,679 lbs per tire; speed rating T = 118 mph). A replacement tire must meet or exceed the load index and speed rating on your vehicle's door-jamb placard.

What this site does not do

A size comparison is not a fitment guarantee. Whether a tire clears your fenders and suspension depends on wheel width and offset, suspension travel, alignment and the specific vehicle. Our checklist flags what to verify; it never certifies a fit. We also do not publish any maker's proprietary fitment database.

Last verified November 12, 2026.