Every tyre carries its size on the sidewall as a short code, something like 205/55 R16 91V. It looks cryptic, but each part tells you one specific thing, and once you can read it you'll never order the wrong tyre.
Where to find your tyre size
You have three places to look, and they should all agree:
- The sidewall of your current tyre. The quickest option. Look for the pattern "three digits, slash, two digits, letter, two digits".
- Inside the fuel filler flap or the driver's door frame. Most cars carry a placard here listing the manufacturer's recommended sizes and pressures.
- Your vehicle handbook. Usually in the technical specifications section.
If the tyres currently fitted don't match the placard, trust the placard. Someone may have fitted the wrong size previously.
What each part of the code means
Taking 205/55 R16 91V apart, piece by piece:
- 205Section width in millimetres, measured tread edge to tread edge
- 55Aspect ratio, the sidewall height as a percentage of the width
- RRadial construction
- 16Wheel rim diameter in inches
- 91Load index, a code for the maximum weight per tyre
- VSpeed rating, the maximum sustained speed
Section width (205)
The width of the tyre in millimetres. A wider tyre puts more rubber on the road, which generally means more grip but slightly worse fuel economy. This has to match what your vehicle is designed for. Going wider than the manufacturer specifies can cause the tyre to foul the arch or suspension on full lock.
Aspect ratio (55)
This one catches people out because it's a percentage, not a measurement. The sidewall height is 55% of 205mm, so about 113mm. A lower number means a shorter sidewall, which gives sharper handling and a firmer ride. A higher number means a taller sidewall, more comfort, and more protection against pothole damage.
Construction (R)
R means radial, so the plies run radially across the tyre. Virtually every modern car tyre is radial, so you'll almost always see R here. You may occasionally see ZR on high performance tyres, which indicates a speed rating above 149mph.
Rim diameter (16)
The wheel diameter the tyre is built to fit, in inches. This is the one number that absolutely cannot vary. A 16 inch tyre will not fit a 17 inch wheel under any circumstances.
Load index (91)
A coded value for the maximum weight each tyre can carry. Some common values:
- 88560 kg
- 91615 kg
- 94670 kg
- 97730 kg
- 100800 kg
- 104900 kg
Never fit a tyre with a lower load index than your vehicle specifies. On vans, SUVs and electric vehicles this matters more than people expect. EVs are heavy, and many require reinforced or Extra Load tyres.
Speed rating (V)
The maximum sustained speed the tyre is rated for:
- T118 mph
- H130 mph
- V149 mph
- W168 mph
- Y186 mph
Match or exceed what your vehicle specifies. Fitting a lower rated tyre than the manufacturer requires is an offence and can invalidate your insurance, even at legal road speeds.
Other markings you might see
Alongside the size, sidewalls carry other codes worth recognising:
- XLExtra Load, or Reinforced. Built to carry more weight than a standard tyre of the same size. Common on SUVs, vans and EVs.
- RFRun Flat, also shown as RFT, ROF or SSR. Reinforced sidewalls let you drive up to 50 miles at up to 50mph after a puncture.
- M+SMud and snow. Self declared by the manufacturer with no test behind it.
- 3PMSFThe three peak mountain snowflake. The tested, certified severe snow rating, and the one that counts in countries with winter tyre laws.
- OEMarks like *, MO, AO, N0 and J mean the tyre was homologated for a vehicle manufacturer's programme. Approval codes, not restrictions.
Do all four tyres need to match?
They need to match across each axle without exception: same size, same speed rating, same load index, ideally the same model. Mixing across an axle affects braking balance and handling in a way you'll feel in an emergency stop.
Front and rear can differ in size if your vehicle was designed that way, which is common on performance and rear wheel drive cars. Check the placard.
Still not sure?
Search by your registration instead. Enter your reg into our tyre finder and we'll match the correct sizes for your vehicle automatically, so there's nothing to decode.