Converting between metric and inch tire sizes

North American tire shelves carry two incompatible ways of describing the same object. A truck tire may be labelled 315/70R17 or 35x12.50R17, and a buyer comparing them has to do arithmetic before the two numbers mean anything to each other.

Going from metric to inches is a formula. Going the other way is a decision.

Metric to inches

Three steps, and the middle one is where the notation earns its reputation.

1. Convert the section width: millimetres ÷ 25.4.
2. Multiply that width by the aspect ratio ÷ 100 to get sidewall height in inches.
3. Double the sidewall and add the wheel diameter.

315/70R17 gives a 12.40 in section, an 8.68 in sidewall, and 34.36 in overall. Do the same for 265/75R16 and you get 10.43 in wide, 7.82 in of sidewall, 31.65 in tall.

Both answers are exact consequences of the code. Whether the tire in your hand measures that is a separate matter, dealt with further down.

Inches to metric is under-determined

Run the arithmetic backwards from 35x12.50R17 and you get: 12.50 in of width, which is 317.5 mm, and a sidewall of (35 − 17) ÷ 2 = 9.00 in, which is 72% of the width.

There is no 317/72R17. Metric sizes exist in the widths and series that manufacturers chose to produce — 305, 315, 325, at 60, 65, 70 — so converting an inch size to metric means picking the nearest size that exists rather than computing one. For a 35-inch tire on a 17-inch wheel, that is 315/70R17, and the substitution costs you 0.64 in of height and 0.10 in of width.

The same exercise from the other direction: 31x10.50R16 versus 265/75R16. The metric size is +0.65 in taller and, slightly counter-intuitively, 0.07 in narrower. They are routinely treated as alternatives, and they differ by 2.10% in rolling diameter — enough to show up on a speedometer, not enough to notice by eye.

"35s" is a class, not a measurement

Here is the trap that costs people money. The 35 in a flotation code is the nominal diameter, and the code is treated as a class label by everyone in the industry, including the manufacturers.

• 35x12.50R17 computes to exactly 35.00 in, because the notation defines it that way.
• 315/70R17 computes to 34.36 in and is universally sold, discussed and fitted as "a 35".
• Physically, a moulded 35-class tire from either notation frequently measures under 34.5 in when new and inflated on its measuring rim, and less again once it is loaded and worn.

So three things called "35s" can span most of an inch. If the number matters — clearance under an arch, ratio arithmetic, matching a spare to a limited-slip axle — the size code has to be run through the arithmetic rather than read as a headline.

The odd corners

• 17.5, 16.5 and 15.5 rims. Older light-truck and trailer wheels came in half-inch diameters, and tires for them are marked accordingly (8.75R16.5). They share nothing with a 16-inch tire but the first digit.
• Bias-ply inch sizes. 7.50-16 is section width in inches, a hyphen for diagonal-ply construction, and wheel diameter. No aspect ratio is stated because the old sizes assumed a fixed, roughly square profile. See construction codes.
• Hard metric. A handful of sizes state the wheel diameter in millimetres too — the TRX and similar systems — and those wheels take nothing else. They are effectively extinct outside restoration work.
• Flotation widths that are not what they say. A 12.50 is a nominal section width in inches, measured on a nominal rim, and the same tire on a wider wheel measures wider. Metric widths carry the same caveat; rim width to tire width has the arithmetic.

Why both notations survived

The metric system arrived with European passenger cars and became the standard for anything speed-rated and road-biased. The inch system stayed where it started: agricultural and off-road tires, where buyers cared about the height of the tire above all else and the marking that stated height directly won.

Light trucks sit in the overlap, which is why one platform's factory size can be metric while the size its owners commonly step up to is flotation, and why the two turn up on adjacent shelves in the same shop.

What "equivalent" means in a fitment guide

Cross-notation equivalence tables — the ones printed on shop walls and built into online size finders — are not claiming the two sizes are the same object. They are claiming the diameters fall within a tolerance of each other, and the tolerance is conventionally about 3% of rolling diameter, because that is roughly where speedometer error stops being trivial.

So 315/70R17 and 35x12.50R17 appear as equivalents on a 1.82% difference, and so do 265/75R16 and 31x10.50R16 at 2.10%. Both pairs are genuinely interchangeable in the arithmetic sense and genuinely different by a measurable amount. Which of the two matters depends on why you are asking.

There is a related convention worth naming: the call size, the nearest half inch of computed diameter, is how enthusiasts talk. A tire computing to 34.36 in is called a 34.5 by that rule and a 35 by the shop. Neither is wrong; neither is a measurement.

Comparing across notations without arithmetic errors

A few habits that prevent the common mistakes:

• Compare computed diameters, never labels. 34.36 against 35.00 is a real comparison; "315s versus 35s" is not.
• Watch the width separately. Diameter and section width move independently between notations, and the narrower-but-taller combination is common.
• Check the service description too. Two sizes that convert cleanly into each other may carry very different load indices, especially where one is P-metric and the other is LT.
• Convert once, at the start. Most fitment arguments dissolve when both candidates are stated in inches to two decimal places.

The size converter does the whole conversion in both directions and shows the nearest real sizes rather than an invented one, and the size code page covers what each element of either marking means before you convert it.