Every tyre has a string of numbers and letters moulded into its sidewall - something like 205/55 R16 91V. To the uninitiated it looks like a code, but each part tells you something important about the tyre. Understanding it means you can order the right tyre with confidence and spot substitutions you shouldn't accept.
Breaking Down 205/55 R16 91V
Let's use one of the most common car tyre sizes as our example: 205/55 R16 91V. Here's what each part means.
205 - Tyre Width
The first number is the tyre width in millimetres, measured from sidewall to sidewall when fitted and inflated. In our example, the tyre is 205mm wide.
Fitting a tyre that is significantly wider or narrower than specified can affect handling, fuel economy and the accuracy of your speedometer. Always match the width unless you've specifically upgraded your wheels.
55 - Aspect Ratio
The second number is the aspect ratio - the height of the tyre's sidewall as a percentage of its width. A 55 aspect ratio means the sidewall height is 55% of 205mm, which is approximately 113mm.
Lower aspect ratios (35-45) give a sportier look and sharper handling but a harsher ride. Higher aspect ratios (65-80) give more comfort and are common on SUVs, vans and budget city cars.
R - Construction Type
The 'R' stands for Radial construction - the internal ply cords run at 90 degrees to the direction of travel. Virtually all modern tyres are radial. You may occasionally see 'D' for diagonal (bias) construction on older or specialist vehicles.
16 - Rim Diameter
This is the diameter of the wheel rim the tyre is designed to fit, measured in inches. The tyre must exactly match the rim diameter - a 16-inch tyre will only fit a 16-inch rim.
91 - Load Index
The load index is a numerical code indicating the maximum weight the tyre can support when inflated to its maximum pressure. 91 corresponds to 615kg per tyre. Always fit a tyre with a load index equal to or higher than the original specification - never lower.
Your vehicle handbook and the tyre placard (usually inside the driver's door jamb) show the minimum load index required.
V - Speed Rating
The speed rating indicates the maximum speed the tyre is designed to sustain. V is rated to 240km/h (149mph). Common speed ratings: T = 190km/h, H = 210km/h, V = 240km/h, W = 270km/h, Y = 300km/h.
Always fit a tyre with a speed rating equal to or above the original specification. Fitting a lower speed-rated tyre can invalidate your insurance.
Other Markings to Know
- XL or Extra Load - the tyre has a reinforced sidewall and can carry heavier loads at higher pressures.
- Run Flat (RF, RFT, ROF) - can be driven on for up to 50 miles at 50mph with no pressure.
- M+S - mud and snow rated; suitable for light winter use.
- The DOT code - the last 4 digits indicate the week and year of manufacture (e.g. 2423 = week 24 of 2023).
Where to Find Your Tyre Size
- On your existing tyres - on the sidewall, usually in a prominent location.
- In your vehicle handbook - the tyre specification section.
- On the tyre placard - usually inside the driver's door frame or fuel filler flap.
- Online tyre lookup tools - enter your registration number to find approved sizes.