How to read your tyre size

Your tyre size is printed on the sidewall, something like 205/55 R16 91V. Each part tells you a specific dimension or rating, and getting them right matters for safety, handling and the law. Once you can read it, ordering the correct tyre, or checking a fitter has quoted the right one, takes seconds. Here's how to decode it.

Breaking down 205/55 R16 91V

Read left to right, each character is a separate piece of information about the tyre's size and limits:

• 205: tread width in millimetres.
• 55: the profile or aspect ratio: sidewall height as a percentage of width (here 55% of 205mm).
• R: radial construction, used on almost all modern car tyres.
• 16: wheel rim diameter in inches.
• 91: the load index (maximum weight per tyre).
• V: the speed rating (the maximum speed the tyre is rated for).

So a 205/55 R16 91V is 205mm wide, with a sidewall 55% of that, on a 16-inch radial wheel, rated to carry 615kg per tyre at up to 149mph. Change any of those numbers and you change how the tyre fits and behaves.

Where to find the right size

Read it off the existing tyre sidewall, or check the placard in the driver's door shut, inside the fuel filler flap, or in the handbook. The car's recommended size is the safe reference, which matters especially if a previous owner fitted the wrong tyres. If the sidewall and the placard disagree, trust the placard, because that's the manufacturer's specification for your car.

Load index and speed rating tables

The load index is a code for the maximum weight each tyre can carry, and the speed rating is a letter for its maximum sustained speed. Neither is printed as a plain number, so here are the values you are most likely to meet on a UK car or van.

The rule is simple: match or exceed what the car's placard specifies, on both. A higher load index or speed rating is fine; a lower one is not, and it can fail an MOT or cause a problem with an insurer.

Note that the speed rating is about the tyre's construction and heat tolerance, not permission to drive at that speed; a V-rated tyre on a car that will never see 149mph is normal.

Other sidewall markings worth knowing

Beyond the size, the sidewall carries a handful of codes that tell you who the tyre was made for, when, and what it can and cannot do. Most are worth a glance when buying or checking a used car.

• The DOT date code: the last four digits of the DOT string, for example 2325, are the week and year of manufacture, so week 23 of 2025. Anything over five or six years old wants a careful age check regardless of tread.
• OE markings: a star, MO, N0, AO, RO1, MOE, K1 and similar mean the tyre was developed for a particular car maker. They are not compulsory replacements, but matching them keeps the car handling as designed.
• Run-flat codes: RFT, SSR, ROF, ZP, DSST, RSC and others all mean run-flat construction. Do not mix run-flat and standard tyres on an axle, see run-flat tyres explained.
• TWI: small arrows or triangles on the shoulder point to the tread wear indicators, raised bars in the grooves at 1.6mm. Tread level with a bar is at the legal limit.
• Max press: the maximum cold inflation pressure the tyre can take, not the pressure to run it at. Your operating pressure is on the car's placard, not the tyre.
• E-mark: a circle with an E and a number shows the tyre is approved for sale in Europe, including the UK.
• XL or Reinforced: an extra-load tyre with a higher weight capacity, common on SUVs, vans and EVs.
• M+S or the 3PMSF snowflake: mud and snow, or certified winter grip, relevant for winter and all-season tyres.
• A directional arrow: the tyre must turn one way, which limits how it can be moved between wheels.

Vans use a slightly different format. A size like 215/65 R16C 109/107 T carries a C for commercial construction and two load indexes, the first for single wheels and the second for twin rears, see van tyres explained. The EU label's noise and efficiency grades are separate again.

Can you change any of the numbers?

You can change the tyre size only within what the manufacturer approves, and you should never go below the specified load index or speed rating. Some changes are routine and safe; others cause problems that only show up at the MOT or in an emergency stop.

Usually fine

• A higher load index or speed rating than the placard.
• An alternative size listed in the handbook or on the placard, such as a smaller winter wheel and tyre combination the maker approves.
• An XL version of the same size, which carries more load at the cost of a slightly firmer ride.

Needs care

• A different width or profile that keeps the rolling diameter within about three per cent of the original. Beyond that the speedometer, ABS and stability control are working from wrong information.
• Different sizes front and rear, which some cars are designed for and most are not.

Do not

• Fit a lower load index because it was cheaper. On an SUV, EV or van the tyre is carrying close to its limit already.
• Fit different sizes or constructions on the same axle. It is an MOT failure and, for construction, illegal; the rules on tyre safety (GOV.UK) set out the basics. A different rolling diameter can also upset the speedometer, ABS and stability systems.
• Rely on the tyre that happened to be on the car when you bought it. Previous owners do sometimes get this wrong.

If you are considering a change in size for winter wheels or different alloys, the safe route is the manufacturer's own list of approved alternatives, then a fitter who will check the clearance on full lock and with the car loaded.

Not sure? We'll match it for you

Tell us your registration or read us the sidewall and we'll source the correct tyre, no guesswork. Our mobile fitting service fits the right size at your door, from our one base at Brent Cross, NW2. London and the Home Counties are closest; further out, call and we will tell you whether we can take the job, and the arrival time is given on the call.