Tire Size Comparison Calculator

How to read the result

The drawing puts both tires on one ground line, so the vertical difference between the crowns is the ride-height change at the axle — half the diameter difference, since the tire grows at the top and the bottom. The bars underneath are section width at the same scale.

The table gives you seven figures. Overall diameter and section width are the ones people ask for. Sidewall height is the one that explains why a wider tire on a smaller wheel can be shorter than a narrow one on a big wheel. Circumference and revolutions per mile are the same fact in two units, and they are what every downstream consequence — speedometer, odometer, engine speed — is derived from.

The speedometer row is the ratio of the two diameters. Nothing else enters it. A speedometer counts wheel revolutions, so if the fitted tire covers more ground per turn, the instrument reads low by exactly the proportion the tire grew.

A worked example you can reproduce

Put 265/70R17 in the left box and 285/70R17 in the right. The 265 measures 31.61 in tall and 10.43 in wide; the 285 measures 32.71 in and 11.22 in. That is +1.10 in of diameter, 3.49% — 0.55 in of extra ride height at the axle, and 0.79 in of extra width.

Revolutions per mile falls from 638.1 to 616.6. At an indicated 70 mph the vehicle is actually doing 72.4, and an indicated 100 miles is 103.5 real miles. Those are the numbers behind the common advice that a 20 mm step "barely matters": it is a third of an inch of ride height and about two and a half miles an hour at highway speed, which is small but is not nothing.

How far to trust the difference figure

The percentage at the centre of the result is the one number here worth being careful about, because it is a difference between two computed values and differences are where small errors become visible ones.

Both diameters are exact consequences of their size codes — that part carries no uncertainty at all. What carries uncertainty is whether a real tire matches its code, and the answer is that it usually does not, quite. Two tires stamped identically can differ by a couple of tenths of an inch between makers, and an aggressive tread commonly measures under its stated size.

The useful consequence is about magnitude, not direction. When this tool reports a difference of several per cent, real-world variation will not overturn it — a 7% step is a 7% step whoever made the tires. When it reports a difference under about one per cent, the two sizes are, for practical purposes, the same height, and which one measures taller on a particular pair of tires is not something arithmetic can tell you. That is the honest reading of a small number here, and it is usually the answer someone comparing a metric size against its imperial near-twin actually needs.

Why real tires drift from their codes — measuring rims, loaded rolling radius, nominal-size tolerance — is set out at how every figure on this site is computed, which is the canonical description of the method for every calculator here.