Every tyre has a string of numbers stamped on its sidewall — like 205/55 R16 91V. Most drivers ignore them, but they carry critical safety information. The wrong tyre size can affect your fuel economy, braking distance, speedometer accuracy, and even your MOT result.
Use the tool above to decode any tyre size in seconds.
What do the numbers on a tyre mean?
Section width (e.g. 205)
The first three digits are the tyre’s width in millimetres, measured from sidewall to sidewall when mounted on a correctly sized rim. A wider tyre gives more grip but can increase fuel consumption and may rub on wheel arches if it’s not the right fitment for your car.
Aspect ratio (e.g. 55)
This is the sidewall height expressed as a percentage of the width. So a 205/55 tyre has a sidewall of 205 × 0.55 = 112.75 mm. Low-profile tyres (35, 40, 45) look sporty but transmit more road noise and are more vulnerable to pothole damage — something to consider on North Wales’s rural back roads.
Construction (R)
R stands for Radial, which is how virtually every modern tyre is built. The steel cords run radially from bead to bead, giving excellent flexibility and heat resistance. You’ll almost never see D (diagonal/bias) on a road car.
Rim diameter (e.g. 16)
This is the wheel size in inches. It must match your alloy exactly — a 16-inch tyre will not fit a 17-inch wheel. When upgrading wheels, you need to recalculate the tyre size to maintain the same overall diameter (known as ‘plus sizing’).
Load index (e.g. 91)
The load index tells you the maximum weight each tyre can support. Index 91 means 615 kg per tyre, giving a total capacity of 2,460 kg for four tyres. Never fit a tyre with a lower load index than your vehicle manufacturer specifies — it’s a safety issue and can void your insurance.
Speed rating (e.g. V)
The letter indicates the maximum sustained speed the tyre is designed for. V = 240 km/h, H = 210 km/h, T = 190 km/h. You should always match or exceed your vehicle’s original speed rating. Fitting a lower-rated tyre is illegal for some vehicles and unsafe for all.
Can I fit a different tyre size?
You can fit an alternative size in some cases — for example, if upgrading to larger alloys — but it must maintain the same overall rolling diameter within 2–3% or your speedometer will read incorrectly, your gearbox ratios will be affected, and it may interfere with your ABS or traction control systems. Always check with a professional before changing size.
Where to find your tyre size
- On the tyre sidewall (most reliable)
- Inside the driver’s door jamb (a sticker showing the recommended sizes)
- In your vehicle handbook
- On the fuel cap (some manufacturers put it there)
If you’re unsure which size to order, send us a WhatsApp photo of your tyre sidewall and we’ll confirm the right replacement before we come out.
This is general guidance to help you decide what to do next — it is not a substitute for a professional inspection. If in doubt, don’t drive on it. Call us and we’ll come to you.