Tyre size decoder
Type the code from your sidewall and see exactly what each part means — width, profile, rim, the weight each tyre can carry and the speed it is rated for.
Found on the sidewall, and on the placard inside the driver’s door frame or the fuel filler flap. The placard is the authority for what your car should be running.
What each part of the code does
A code like 205/55 R16 91V looks like a serial number and is really a specification. Every element has a job, and getting one of them wrong has consequences ranging from a harsh ride to an invalidated insurance claim.
- 205 — section width in millimetres, measured across the tread.
- 55 — aspect ratio. The sidewall height is 55 per cent of the width, so about 113 mm. Lower numbers mean shorter, stiffer sidewalls.
- R — radial construction.
- 16 — rim diameter in inches.
- 91 — load index, a coded maximum weight per tyre. 91 is 615 kg.
- V — speed rating, a coded maximum speed. V is 149 mph.
What you can change, and what you should not
The correct specification is in your handbook and on the placard inside the driver’s door frame or behind the fuel filler flap. Use that rather than copying whatever is fitted now — previous owners fit the wrong thing more often than most people expect.
- Load index: never go lower. A tyre carrying more than it was built for builds heat, and heat is what destroys tyres at speed. Higher is generally fine, though a much stiffer tyre can make the ride noticeably harder.
- Speed rating: never go lower. It is not automatically illegal, but it is unsafe and may allow an insurer to decline a claim. The usual exception is a winter tyre, where a lower rating is permitted on some vehicles if a sticker showing the maximum speed is displayed.
- Keep sizes matched across an axle. Different grip left to right makes braking and cornering unpredictable.
- Check the handbook before mixing brands on four-wheel drive. Some transmissions are sensitive to small differences in rolling radius.
- Changing overall diameter affects your speedometer and, on many cars, the ABS and stability systems. Stay within the specified sizes.
The rest of the sidewall — XL, run-flat codes, the snowflake symbol, the date code — is covered in our full guide to tyre markings, and you can check how old a tyre is with our tyre age checker.
Read us the code and we will bring it to you
Our mobile tyre fitting service supplies, fits and balances at your home, work or the roadside across North Wales. Read the numbers off the sidewall when you call and we will confirm exactly what fits.
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Questions, answered
What does 205/55 R16 91V mean?
205 is the section width in millimetres. 55 is the aspect ratio, so the sidewall height is 55 per cent of the width, about 113 mm. R is radial construction. 16 is the rim diameter in inches. 91 is the load index, equal to 615 kg per tyre. V is the speed rating, equal to 149 mph.
Where do I find my correct tyre size?
In the vehicle handbook and on a placard, usually inside the driver's door frame or behind the fuel filler flap. Use that rather than the size currently fitted, because a previous owner may have fitted something incorrect.
Can I fit a different size tyre to my car?
Only within the sizes the manufacturer approves. Changing the overall diameter affects speedometer accuracy and can interfere with ABS and stability control. Width and profile also change handling, and the load index and speed rating must still meet the vehicle's requirement.
What happens if the load index is too low?
The tyre is being asked to carry more weight than it was designed for. That builds heat, which is the main cause of tyre failure at speed, and it can also affect an insurance claim. Never fit a lower load index than your vehicle specifies.
Is a higher speed rating a problem?
No, fitting a higher speed rating than your car requires is acceptable. Tyres with very high ratings can wear faster and perform less well in cold conditions, but they are not unsafe. Fitting a lower rating than specified is the thing to avoid.
What does the R in a tyre size mean?
R stands for radial construction, which is used on virtually every modern car tyre. You may occasionally see D for diagonal or bias-ply construction on trailers and classic vehicles. Radial and cross-ply tyres must never be mixed on the same axle.