HomeMethodology

Methodology and sources

Data last verified 29 October 2026. No figure on this site is typed in by hand: each one is calculated from the size code with constants from a saved copy of a published source, or read from a saved copy of a maker's table.

How the figures are calculated

Metric sizes (like 285/70R17): the sidewall is the width times the aspect ratio, as a percentage, and the overall diameter is two sidewalls plus the wheel diameter.
Flotation sizes (like 33x12.50R17): the height and width are the inches in the code, and the sidewall is half of the height minus the wheel. Flotation sizes state their height and width in inches, and those numbers are nominal: makers aim at them, and real tires of the same size measure a little differently.
Units: an inch is exactly 25.4 mm and a mile exactly 1.609344 km (NIST), so a mile is 63,360 inches.
Revolutions per mile are a mile divided by the circumference. They are labelled calculated: makers publish their own, measured, figures, and in every one of the 256 maker figures in the sources the measured number is higher than the calculated one.
Speedometer: true speed is the indicated speed times the new diameter over the old one, for a speedometer that read true on the old size. The odometer error is the same ratio turned over.
Ground clearance at the axle changes by half the diameter change, unloaded.
Rounding: calculations run at full precision and round only for display. A difference shown beside two rounded figures is the difference of those figures, so each row adds up; percentages use full precision.

How the calculations are checked

Tests reproduce every figure that Performance Plus Tire, SimpleTire, tiresize.com and TacomaWorld publish for the sizes in the sources, at each publisher's own rounding. tiresize.com rounds pi to 3.14 for circumference and revolutions, which moves its revolutions per mile by 1 on some sizes; the tests allow for that convention and nothing else.

Makers' measured diameters are checked too: across 13 maker size tables, the largest gap between a measured diameter and the calculated one is 1.67%. 4 rows in one maker's table print a diameter far from the size (by more than the 3% the check allows); those are recorded as the source's errors and left out.

Size catalogue and inch classes

The size chart, the inch-class pages and the nearest-size lists use a catalogue of 154 sizes: those that at least two of the makers' size tables list. Any other size still works in the calculator. A size's inch class is its calculated diameter rounded to the nearest inch; a flotation size goes by the number in its code.

What the site doesn't cover

These are size-code figures only. Whether a size fits, clears and carries your load depends on your vehicle and the tire, so check with a tire shop. Load index, load range, speed rating and the P or LT prefix don't change the size-code geometry, so the calculator ignores them.

Sources

Rules

R0. Not a figure: a name, a size code or a flag, with the quote that supports it.
R1. Unit constants come from NIST SP 811 Appendix B.8 only (inch and mile, both exact); figures derived from them carry their formula.
R2. Metric sizes: sidewall = width x aspect / 100; diameter = 2 x sidewall + rim. The rule needs two independent sources that agree.
R3. Flotation sizes: diameter and section width are the stated inches; sidewall = (diameter - rim) / 2; results are nominal.
R4. Prefixes (P, LT, ST, ZR) and service descriptions never enter the maths.
R7. A maker's measured diameter more than 3% from the calculated diameter fails the band check (tests/makers.test.js); every gap is logged. A row that is off because the maker's own table prints a wrong figure is listed in SOURCE_ERRORS with its quote, logged, and left out.
R8. A size is in the catalogue if 2+ snapshotted maker size tables list it.
R9. Makers' measured diameters and revs per mile are reported as measured, beside the calculated figures, never in their place.
R11. Ground clearance change = half the diameter change, at the axle, unloaded, from a quoted source.

Updates

The sources are re-checked once a year and whenever a correction is reported. Every visible change is listed on the changes page.

Methodology and sources

How the figures are calculated

How the calculations are checked

Size catalogue and inch classes

What the site doesn't cover

Sources

Unit constants

What a size code means

Retailers' calculated figures (the calculations must reproduce them)

Calculators' published figures (the calculations must reproduce them)

Makers' size tables (measured figures and the size catalogue)

Rules

Updates