The tire size code, digit by digit

A tire's size marking is a specification, not a description. It tells you what the casing was designed to be, in a notation agreed internationally, and once you can read it you can work out every dimension that follows.

There are two notations in common use, and North American light trucks are sold in both.

Metric notation: LT285/70R17

Read left to right.

LT — service type. Optional. P means passenger, LT light truck, ST special trailer, T temporary spare. A bare code with no letter is a Euro-metric passenger tire. The prefix does not change the geometry at all: an LT-marked tire is the same shape as an unmarked one of the same numbers. What it changes is the casing construction and the reference inflation pressure, and therefore the load index. This is the single most consequential element that people skip.

285 — nominal section width in millimetres. The widest point of the inflated casing, sidewall bulge included — not the tread. Measured at a specified nominal rim width; mount the same tire half an inch wider and it gains roughly 0.2 in of section width.

70 — aspect ratio. The sidewall height as a percentage of the section width. Here the sidewall is 70% of 285 mm, which is 199.5 mm, or 7.85 in. This is the element that trips people: it is not a dimension, it is a ratio, so a "70-series" tire is only tall if it is also wide.

R — construction. R is radial. D, or a plain hyphen, is diagonal bias. B is bias-belted. ZR indicates a Z speed rating carried in the size position, a hangover from when Z was the top of the scale.

17 — wheel diameter in inches. The one imperial number in a metric code, because wheels never went metric.

To get overall diameter: the sidewall appears twice, once above the wheel and once below, so it is 17 + 2 x 7.85 = 32.71 in.

Flotation notation: 33x12.50R17

Read left to right again, but the notation answers different questions.

33 — nominal overall diameter in inches. Printed directly, which is the whole appeal of the notation. It is nominal: a tire marked 35 frequently measures nearer 34.5 in, because the number is a class rather than a specification.

12.50 — section width in inches. Same definition as the metric width, different unit.

R17 — construction and wheel diameter. Identical meaning to metric.

There is no aspect ratio printed. Derive it: sidewall height is (33 - 17) / 2 = 8.00 in, and 8.00 / 12.50 = 64. So a 33x12.50R17 is a 64-series tire and nothing on it says so. This is why comparing flotation sizes by "series" is impossible at a glance: 33x12.50R17 is a 64, 35x12.50R17 is a 72, and 37x12.50R17 is an 80, all on the same nominal width and wheel.

What follows the size

The service description — something like 121/118S — is the load index and speed symbol, and it constrains what the tire can do more than the size does. Where two load-index numbers appear separated by a slash, the first is single-wheel and the second dual-wheel.

The rest of the sidewall

The size code and service description are the specification. Around them sit the DOT code with its four-digit week-and-year date, the UTQG treadwear, traction and temperature grades, the maximum load and pressure statement, the M+S and three-peak snowflake markings, and the manufacturer's own model name — the only part of the sidewall that is marketing.

A note on spelling

British English spells it tyre; American English spells it tire. The two words mean exactly the same thing and the markings are identical worldwide, because the notation is standardised internationally rather than nationally. This site uses the American spelling throughout, including in its addresses, because that is how the majority of people searching for this arithmetic write it.

The one thing the code cannot tell you

Whether the tire fits your vehicle. The code gives you geometry, exactly and completely. Clearance, load rating against your placard, and road-legality where you live are three separate questions about three other objects.

Two codes read side by side

Take 265/70R17 and 33x12.50R17, both extremely common on the same wheel.

The metric one decomposes cleanly: 265 mm of section width, a sidewall 70% of it at 7.30 in, a 17-inch wheel, and therefore 31.61 in overall. The imperial one states 33.00 in and 12.50 in directly and leaves you to work out that its sidewall is 8.00 in and its aspect ratio 64.

Everything a buyer would want to compare is present in both codes, but each notation hides the thing the other states plainly. Read one of each often enough and you stop noticing; read a shelf full of mixed notation and it is genuinely hard work.

Why the aspect ratio is the element that catches people

Because it is the only element that is not a dimension. Change 265 to 285 and the sidewall grows too, because 70% of a bigger number is a bigger number — the tire gets taller as well as wider, which is not what "wider tire" sounds like it should mean.

The consequence is that two sizes with the same aspect ratio grow together, and two sizes with the same width but different aspect ratios differ only in height. If you want to change one dimension at a time, that is the lever: hold the width and move the aspect ratio, or hold the aspect ratio and move the width.

The prefix, once more, because it is the one that costs money

P and LT are the two that matter in North America. They are not descriptions of the vehicle the tire goes on — they are the standard the tire was built and rated to. An LT tire has a heavier casing and a much higher reference inflation, and its load index reflects that. A P-metric tire fitted to a light truck has its load capacity de-rated in the calculation the vehicle manufacturer performs, which is one of the reasons the placard says what it says.

Two tires on a shelf, both marked 285/70R17, one with LT and one without, are not interchangeable in any sense except the geometric one. Everything on this site is about the geometric sense, which is exactly why this paragraph is here.

Where to go next

The load index and speed rating reference explains the service description that follows the size. The plus-sizing page explains how to change wheel diameter while holding rolling diameter. And the comparison tool will take any two codes, in either notation, and show what changes between them.