Where do the numbers come from?

They are calculated from the tire size code itself using standard geometry, not copied from a lookup table. That means every one of the comparison pages on this site can be reproduced by hand from the formulas published above.

How accurate are the results?

Treat them as accurate to within roughly 1%. The size code gives nominal dimensions; the molded dimensions of a real tire vary by a few millimetres between manufacturers and models, and a mounted tire under load has a smaller rolling radius than its free radius.

About TireSizeCheck

TireSizeCheck answers one question as precisely as it can be answered: if you put a different size tire on your vehicle, what actually changes? It covers 150 common tire sizes and 2,156 size-to-size comparisons.

It is an independent reference. It is not affiliated with any tire manufacturer, retailer or vehicle maker, and nobody pays for placement, ordering or verdicts on this site.

How the numbers are calculated

Every figure on this site is derived from the tire size code itself. There is no lookup table and no estimation, which means any page here can be reproduced by hand:

Sidewall height: tread width x aspect ratio / 100. A 225/45R17 has a sidewall of 225 x 45% = 101.25 mm.
Overall diameter: wheel diameter + two sidewalls. For 225/45R17: 17 in (431.8 mm) + 2 x 101.25 mm = 634.3 mm, or 24.97 in.
Circumference: pi x overall diameter.
Revolutions per mile: 63,360 in / circumference in inches.
Speedometer error: the ratio of the two overall diameters. A tire 3% larger turns 3% fewer times per mile, so an indicated 60 mph is really about 61.8 mph.

The 3% and 5% rules

Fit verdicts on this site use the widely applied diameter thresholds: under 3% is treated as safe, because speedometer, odometer, ABS and stability-control calibration all assume the factory rolling circumference; 3–5% is workable but warrants a clearance check and leaves a measurable speedometer error; over 5% is not recommended without recalibration. A swap that stays within 3% on diameter but adds 20 mm or more of tread width is also flagged, because width has its own clearance consequences.

Accuracy and limits

The size code gives nominal dimensions. Real molded dimensions vary by a few millimetres between manufacturers and models, and a mounted tire under load has a smaller rolling radius than its free radius. Treat results as accurate to within roughly 1%, and confirm fitment against your vehicle’s tire and loading information placard — on the driver’s door jamb — or with a qualified tire professional. This site does not model load index, speed rating, wheel width range or offset, all of which matter for a real fitment decision.

Common questions

Who runs TireSizeCheck?: TireSizeCheck is an independent reference site. It is not affiliated with any tire manufacturer, retailer or vehicle maker, and no manufacturer pays for placement, ordering or verdicts anywhere on the site.
Where do the numbers come from?: They are calculated from the tire size code itself using standard geometry, not copied from a lookup table. That means every one of the comparison pages on this site can be reproduced by hand from the formulas published above.
How accurate are the results?: Treat them as accurate to within roughly 1%. The size code gives nominal dimensions; the molded dimensions of a real tire vary by a few millimetres between manufacturers and models, and a mounted tire under load has a smaller rolling radius than its free radius.

Sources

49 CFR § 571.110 — Tire selection and rims, the federal standard behind the tire and loading information placard.
NHTSA TireWise — Tire safety ratings and awareness, for tread depth, load ratings and maintenance guidance.

Affiliate disclosure

TireSizeCheck is reader-supported. Some outbound links to tire retailers may be affiliate links, which can earn us a commission at no extra cost to you. Affiliate relationships never influence the calculations, the ordering of results or the fit verdicts on this site — the verdicts are produced by the formulas above and nothing else.

Last updated September 2026.