Tyre size explained (UK): reading 205/55 R16 91V

What every part of a UK tyre size means, from 205/55 R16 91V to XL, C, ZR, M+S and T spares, plus the diameter maths and the same-axle MOT rule.

205/55 R16 91V, piece by piece

A tyre size like 205/55 R16 91V reads as: 205 mm wide, a sidewall 55% as tall as that width, radial construction, for a 16-inch rim, with load index 91 and speed symbol V. Everything a fitter needs to pick the right tyre off the rack is in those twelve characters.

UN ECE Regulation 30, the standard behind the E-mark on European and UK tyres, defines each piece. Section width is in millimetres. The aspect ratio is the section height divided by the section width, times 100. The letter R goes in front of the rim diameter on a radial tyre. Its own worked example is 185/70 R 14, with a space before the rim size that most sidewalls now leave out.

The mix of units confuses customers. Millimetres for width. A percentage for height. Inches for the rim. It's a historical leftover, and it isn't going away.

Most people in the UK call the second number the profile, not the aspect ratio. "Low profile" just means a small number there: a 35 or 40 on an 18-inch or 19-inch wheel.

The other letters you'll find

The basic size is the easy bit. Then come the extras. Read all of them.

• XL or REINFORCED: extra load version of the same size, with a higher load index. A car that left the factory on XL should go back on XL.
• C after the rim size, as in 215/65 R16C: a commercial van tyre, often with two load indices (single and twin fitment).
• ZR in place of R, as in 225/40 ZR18: a radial rated over 240 km/h. Regulation 30 makes ZR compulsory above 300 km/h, where the service description goes in brackets, like (95Y).
• T in front of the width, as in T125/70 R17: a T-type temporary-use spare. Regulation 30 requires it to be marked INFLATE TO 420 kPa (60 psi).
• M+S (or M.S or M&S): the maker's own mud and snow marking. Michelin points out that only the three-peak mountain snowflake (3PMSF) symbol shows a tyre passed standardised winter testing.
• Run-flat markings vary by brand, so read the maker's literature rather than guessing from a code.

The load index and speed symbol are covered in full on our load index and speed rating chart.

Overall diameter: the maths

To compare two sizes, work out the overall diameter in millimetres. Sidewall height is width times profile. Double it, because there's a sidewall above and below the rim, then add the rim size converted to millimetres (inches times 25.4).

For 205/55 R16: 205 × 0.55 = 112.75 mm of sidewall. Double it. That's 225.5 mm. The rim is 16 × 25.4 = 406.4 mm. Total: 631.9 mm.

This is what plus-sizing is: a bigger rim, a lower profile, the same overall diameter. Keep the diameter close and the speedometer, gearing and ride height stay close too. A change of a percent or two shows up on the speedo; a few percent and the car sits noticeably different. The maths only tells you whether a size is close. It doesn't tell you whether it clears the arches, the brake callipers or the suspension on full lock, and it doesn't make the size one the car maker lists. Check the handbook.

The rules that matter for size

Two documents decide most of what a UK fitter can and can't do with sizes.

The MOT inspection manual lists these as major defects:

• tyres on the same axle of different sizes;
• tyres on the same axle of different structure, such as radial and cross-ply;
• a tyre fouling part of the vehicle;
• a tyre not fitted in line with the maker's sidewall instructions.

The manual adds that run-flat and conventional tyres can be mixed on an axle, though it isn't recommended. An asymmetric tyre fitted the wrong way round isn't a reason for rejection either, only advice to the presenter. Fix it anyway.

Regulation 27 of the Construction and Use Regulations bans using a tyre that is unsuitable for the vehicle's use or for the tyres fitted to its other wheels. And the GB-retained Regulation (EU) No 458/2011 sets the approval floor: every tyre's load capacity must be at least half the maximum permitted mass of the most heavily loaded axle, with a speed symbol compatible with the car's top speed. It also says vehicles other than cars and light vans (categories M1 and N1) can't be fitted with a temporary-use spare at all.

What that means at the counter: a pair across an axle, same size, same construction. Always. If a customer wants one tyre and the other side of the axle turns out to be a different size, or a different construction that somebody else fitted years ago without anyone noticing, sell two.

How customers say it

Nobody reads a size the way it's printed. You'll hear "two-oh-five fifty-five sixteens", see a photo with a thumb over the load index, or get a text saying 2055516. On the phone, "a sixteen-inch tyre for a Focus" is a start, not a size; check the sidewall or the handbook, because the same model can run several sizes depending on year and trim.

A decent stock search should treat 205/55 R16, 205/55/16 and 2055516 as the same thing. Read the date code before you quote, using our tyre date code guide, and check depth on a part-worn with the tread depth chart.

DuroBay is a free app for tyre shops that keeps stock and reads the size off a photo of the sidewall, so all three ways of typing it find the same tyre.