How to Read a Tire Size: 225/45R17 Decoded

A tire size looks cryptic but it’s just four facts in a row. Take 225/45R17: that’s a tire 225 mm wide, with a sidewall 45% of that width, of radial construction, made to fit a 17-inch wheel. Read it once and you can decode any tire on the road — and work out the one measurement that actually matters for fitment: the overall diameter.

The four parts, left to right

Width (225). The section width in millimetres — the tire’s width across the widest part of the tread. Bigger numbers mean a wider tire.

Aspect ratio (45). The sidewall height as a percentage of the width, not an absolute number. A 45 means the sidewall is 45% of 225 mm = 101 mm tall. This is the part people most often misread: a “45-series” tire isn’t 45 of anything fixed — change the width and the same 45 gives a different sidewall height.

Construction (R). R for radial, the standard for virtually all modern passenger tires. You’ll occasionally see ZR (associated with high speed ratings) or, on classic vehicles, D or B for bias-ply.

Rim diameter (17). The wheel diameter the tire fits, in inches. This is the metric-and-imperial mash-up that trips people up — the width and sidewall are millimetres, but the rim stays in inches.

The number that isn’t written on the tire: overall diameter

The single most useful figure — for speedometer accuracy, gearing and whether a tire will fit — is the overall diameter, and it’s not printed on the sidewall. You calculate it:

Overall diameter = rim diameter + 2 × sidewall height

For 225/45R17, converting the rim to millimetres:

17 × 25.4 + 2 × (225 × 0.45) = 431.8 + 202.5 = 634.3 mm ≈ 25.0 inches

There are two sidewalls — one at the top, one at the bottom — which is why the sidewall height is doubled. From the diameter you can also get the circumference (π × diameter) and the revolutions per mile, which is what gearing and speedometer calculations actually use.

Why the overall diameter matters for fitment

Your speedometer and odometer are calibrated to the original tire’s diameter. Fit a taller tire and you travel farther per revolution, so you’re actually going faster than the speedometer shows and your odometer under-counts. A shorter tire does the reverse. The common guideline is to keep any size change within about ±3% of the original overall diameter — beyond that the errors grow and you risk clearance or gearing problems.

This is also the idea behind plus-sizing: fitting a larger-diameter wheel with a lower-profile tire (say 225/45R17 → 245/40R18) while keeping the overall diameter about the same. The look changes; the rolling diameter shouldn’t.

The tire size calculator decodes any size into its real dimensions, and the tire size comparison tool puts two sizes side by side and shows the exact speedometer error from switching — all in your browser, nothing uploaded.

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The overall-diameter, circumference and revolutions-per-mile figures here are exact geometry from the standard tire-code formula. Always match the size on your vehicle’s door-jamb placard, and confirm clearance before changing sizes.