How to read a tire size

A tire size is one string doing five jobs at once. Once you know which part is which it stops being a code and becomes a short list of measurements — and every one of them has to match, because a tire that is the wrong size does not fit badly, it does not fit at all.

Reading it left to right

Take 225/45R17 94V. Every passenger size follows this shape.

225 — section width, in millimetres

The width at the widest point of the tire, which is the bulge of the sidewall, not the tread. A 225 is nine inches wide across the carcass; the part actually touching the road is narrower than that.

This is a nominal figure. Two 225s from different manufacturers can measure a few millimetres apart, which matters when you are close to rubbing.

45 — aspect ratio, as a percentage

The sidewall height expressed as a percentage of the width. A 45 means the sidewall is 45% of 225mm — about four inches. This is why the same-looking number means different things at different widths: a 45 on a 285 is a taller sidewall than a 45 on a 225.

Lower is shorter. A shorter sidewall flexes less, which sharpens steering response and makes the ride firmer, and it leaves less rubber between your wheel and a pothole.

R — construction

Radial. The plies run across the tire from bead to bead, with belts under the tread. It is effectively universal on modern passenger vehicles; you will still see D for diagonal bias on some trailer and utility tires.

17 — wheel diameter, in inches

The one number with no tolerance at all. A 17" tire mounts on a 17" wheel and nothing else. Mixed units in one size — millimetres for width, a percentage for the sidewall, inches for the wheel — is a historical accident nobody has been willing to fix.

94 — load index

A lookup code, not a weight. 94 means 1,477 lb per tire at full inflation pressure. Four of them is the most the set can carry, and your vehicle's door placard states the minimum index it was engineered around.

Light truck tires often carry two: 121/118S is 121 running as a single tire and 118 when it is one of a dual pair, because a tire in a dual carries less.

V — speed rating

The sustained speed the tire is certified to run at — V is 149 mph. Nobody drives at 149 mph, which makes this look like a number to ignore, and it is not: the rating reflects how the tire is built. A higher rating usually means a stiffer casing and more heat tolerance, which is what you are actually buying on a long hot motorway run at legal speeds.

The markings after the size

• XL or Extra Load — a reinforced casing that carries more, at a higher inflation pressure. Not interchangeable with a standard-load tire unless the pressure goes up with it.
• Load range B/C/D/E — the light-truck equivalent, historically a ply count. E is a stiff, high-pressure casing meant for load, and it rides like one.
• RFT, ROF, ZP, SSR, DSST — run-flat, in five manufacturers' house notation. Drivable for a limited distance with no air.
• M+S and the three-peak mountain snowflake — mud and snow branding, and actual severe-snow certification. Only the second one is a test result.

The sizes that do not follow the pattern

Flotation sizes, common on light trucks, state everything in inches and in a different order: 33x12.50R15 is 33 inches tall and 12.5 inches wide on a 15" wheel. There is no aspect ratio because the height is given directly. The decoder above handles them.

A leading letter is the tire's intended service: P for passenger, LT for light truck, ST for special trailer. An LT tire in a passenger size is a heavier, stiffer tire — it is not a free upgrade.

Where to find yours

Three places, in order of trustworthiness: the placard in the driver's door jamb, which states what the vehicle was designed for; the owner's manual; and the sidewall of the tire currently fitted, which tells you what somebody last put on it. Those are not always the same, and when they differ the placard is the one to trust.