Tire Size Comparison

About this calculator

Put two tire sizes side by side and see exactly what changes: overall diameter, section width, sidewall height, circumference, revolutions per mile, and how far the speedometer will drift. Both tires are drawn to scale with the wheel at your real rim size, so the picture is the arithmetic rather than an illustration of it.

How to read your results

The verdict line is the headline: the percentage change in overall diameter, coloured against the ±3% the trade works to. Everything else follows from that one number. Half the diameter change becomes ride height and the other half closes the gap to the wheel arch, so a +28 mm tire sits 14 mm higher and 14 mm nearer the bodywork. The speedometer figure is a pure diameter ratio, which is why it holds in mph and km/h without conversion. At true scale a few percent is only a few millimetres, so the drawing has a zoom: press “Zoom to the difference” and it frames the gap between the two tires however small it is.

How it's calculated

Overall diameter follows UN Regulation No. 30 §6.1.2.1: D = d + 2H, where d is the rim diameter and H the nominal section height. A metric code states H as a percentage of the section width — for 265/70R17, H = 0.70 × 265 = 185.5 mm, so D = 17 × 25.4 + 2 × 185.5 = 802.8 mm. A standard (flotation) code such as 33x12.50R15 states the diameter and width directly in inches, so H is whatever is left: (33 − 15) ÷ 2 = 9 in. Circumference is π × D, and revolutions per mile is 1,609,344 ÷ C, using the inch and the mile as NIST defines them — 25.4 mm and 1,609,344 mm, both exact. Speedometer error is the ratio of the two diameters. Note that revolutions per mile here is a free geometric figure computed from nominal diameter; manufacturers publish it from measured rolling circumference under load, which is slightly smaller.

Worked example

Compare 265/70R17 against 285/70R17 — 20 mm wider on the same 17-inch wheel.

Overall diameter goes from 802.8 mm to 830.8 mm, a gain of 28.0 mm or +3.49%. The car sits 14 mm higher and loses 14 mm of wheel-arch clearance. At an indicated 60 mph you are really doing 62.1, and over 1,000 miles the odometer records only 966. That is past the usual 3% guideline, so clearance wants checking at full lock and full compression.

Frequently asked questions

Is the ±3% rule an actual regulation?

No. Staying within about 3% of the original diameter is industry practice and a sound rule of thumb, and this page uses it to colour the verdict — but no statute states it. UN Regulation No. 30 does define a tolerance, and it is a different thing: a manufacturing band for one tire against its own designation, not a limit on how far a replacement may deviate from the original size. Its coefficients also multiply 2H — twice the section height — rather than the overall diameter, so expressing that band as a fixed percentage of diameter is wrong; the figure moves with the aspect ratio.

Will a bigger tire make my speedometer read wrong?

Yes, unless it is recalibrated. A taller tire covers more ground per revolution, so the speedometer under-reads and you travel faster than it shows; a shorter tire does the opposite. The error is simply the ratio of the two overall diameters, so a 3.5% taller tire reads 3.5% low at every speed. Because it is a ratio, the same percentage applies whether the dial is in mph or km/h. The odometer drifts by the same proportion.

What is the difference between 265/70R17 and 33x12.50R15?

They are two notations for the same kind of measurement. The metric code gives section width in millimetres, then the sidewall as a percentage of that width, then the rim in inches. The standard or flotation code, common on trucks and off-road builds, states the overall diameter and the section width directly in inches. Neither is more precise than the other — this calculator reads both and will re-express your tires in either without changing them.

Why do two tires that differ by 3% look identical in the diagram?

Because at true scale they nearly are. Three percent of an 800 mm tire is about 24 mm — roughly the width of two fingers against something the size of a dustbin lid. Every comparison tool that draws two tires next to each other runs into this, which is why the drawing here has a continuous zoom and a button that frames the gap between the two edges directly. The number is small; seeing that it is small is part of the answer.

Can I fit a wider tire on the same wheel?

Often, within limits, but section width and rim width are separate measurements and each rim width suits a range of tire widths. This calculator compares tire geometry only — it does not model rim width, offset, backspacing or suspension clearance. Use the sizes-that-fit list as a shortlist of diameters that will not upset your speedometer, then confirm the width against the wheel and the arch before buying.

Does a taller tire change my gearing?

Effectively, yes. A taller tire turns fewer times per mile, so at any given road speed the engine turns more slowly — the same result as fitting a taller final drive. That usually means slightly lower cruising revs and slightly worse acceleration. The percentage change in revolutions per mile is the percentage change in engine speed at a fixed road speed.

Sources

• eur-lex.europa.eu/legal-content/EN/TXT/HTML/?uri=CELEX:42008X0730(01) — European Union
• www.nist.gov/pml/special-publication-811/nist-guide-si-appendix-b-conversion-factors/nist-guide-si-appendix-b9 — National Institute of Standards and Technology

Reviewed by the YouCalc Team · Last reviewed May 24, 2026