How to Read Your Tyre Size (in 60 Seconds)

By Treadside, mobile tyre fitters in North LondonUpdated October 2026

Where to find it

Your tyre size is moulded into the sidewall of every tyre, on the outer face you can see without crawling under the car. You will usually also find it on a sticker inside the driver's door jamb or the fuel filler flap, and in the handbook. The sidewall is the source of truth: it tells you what is fitted right now.

Decoding the numbers

Take a common size: 205/55 R16 91V.

205 is the tyre's width in millimetres.
55 is the profile: the sidewall height as a percentage of the width. A lower number means a lower sidewall.
R means radial construction, which covers virtually every modern tyre.
16 is the wheel rim diameter in inches.
91 is the load index: the maximum weight the tyre is rated to carry. Don't go below what your car requires.
V is the speed rating. Each letter corresponds to a maximum sustained speed the tyre is built for.

Other markings worth knowing

XL or Reinforced: an extra-load version of the size, common on heavier cars and EVs.
Run-flat markings: brands use their own codes, such as RFT, ROF, SSR and ZP. If your car came on run-flats and has no spare, mention it when you message us. See can run-flat tyres be repaired?
M+S and the three-peak snowflake symbol (3PMSF): mud and snow, and certified winter performance, found on all-season and winter tyres.
The date code: four digits after "DOT", giving the week and year the tyre was made. 2423 means week 24 of 2023.

Two catches that cause wrong orders

Front and rear can be different sizes. This is common on rear-wheel-drive and performance cars, and almost universal on motorcycles. Check both ends.

The same car comes in many sizes. Two identical-looking hatchbacks can wear different sizes depending on trim and wheel option, which is why we work from your sidewall or reg and not the model name.

The easy way

Take a photo of the sidewall and send it on WhatsApp with your postcode. We read the size, the load and speed ratings and any run-flat marking for you, and come back with an all-in quote.

We come to you