Decode tyre markings into actual dimensions and specifications
Every tyre fitted to a UK vehicle carries a standardised marking on its sidewall that describes its dimensions, construction, and performance capabilities. A typical marking like 205/55R16 91H contains a wealth of information, but decoding it requires understanding the metric, imperial, and percentage-based measurements used in the tyre industry. The tyre size marking system was established by the European Tyre and Rim Technical Organisation (ETRTO) and is used across Europe and the UK. The first number (205) is the section width in millimetres -- the widest point of the tyre from sidewall to sidewall. The second number (55) is the aspect ratio -- the sidewall height expressed as a percentage of the width. The letter R indicates radial construction (virtually all modern tyres), and 16 is the rim diameter in inches. Understanding your tyre dimensions matters for several practical reasons. When replacing tyres, you must fit the correct size for safe handling, accurate speedometer readings, and legal compliance. If you are considering upgrading to larger alloy wheels, you need to calculate whether the alternative tyre size maintains a similar overall rolling diameter. This calculator converts your tyre markings into actual measurements in both millimetres and inches, helping you compare sizes, check speedometer accuracy, and make informed tyre purchasing decisions.
To decode your tyre size: 1. Enter the tyre width in millimetres. This is the first number in your tyre marking, found on the sidewall of your current tyres. Common widths range from 155mm for city cars to 315mm for high-performance sports cars. The most popular UK sizes are 185, 195, 205, and 225mm. 2. Enter the aspect ratio as a percentage. This is the second number in the tyre marking, after the forward slash. It represents the sidewall height as a percentage of the width. A lower aspect ratio means a shorter sidewall and sportier handling but a firmer ride. Common values range from 35% on sports cars to 65% on family saloons. 3. Enter the rim diameter in inches. This is the number after the R in the tyre marking. Common UK rim sizes range from 14 inches for older or budget cars up to 22 inches for premium SUVs. The most popular sizes are 15, 16, and 17 inches. 4. Select the speed rating. This is the letter at the end of the full tyre marking (after the load index number). It indicates the maximum speed the tyre is certified to handle. H (210 km/h) is the most common for family cars. 5. Review the calculated dimensions. The sidewall height shows the actual height of the tyre rubber above the rim. The overall diameter is crucial for comparing alternative sizes -- it should stay within 3% of the original to avoid speedometer and handling issues. The circumference and revolutions per mile are useful for understanding how tyre size affects your speedometer accuracy.
The tyre size calculations convert between the mixed measurement systems used in tyre specifications: **Sidewall Height:** Sidewall height (mm) = tyre width (mm) x aspect ratio / 100 For a 205/55R16: sidewall = 205 x 55 / 100 = 112.75mm. This is the height of the rubber between the rim edge and the road surface. **Overall Diameter:** Overall diameter (mm) = (rim diameter x 25.4) + (2 x sidewall height) The rim diameter is converted from inches to millimetres (1 inch = 25.4mm), then two sidewall heights are added (one for the top and one for the bottom of the tyre). For 205/55R16: overall = (16 x 25.4) + (2 x 112.75) = 406.4 + 225.5 = 631.9mm. **Circumference:** Circumference (mm) = PI x overall diameter This is the distance the tyre covers in one complete revolution. For our example: PI x 631.9 = 1,985mm or approximately 1.985 metres per revolution. **Revolutions Per Mile:** Revolutions per mile = 1,609,344 / circumference (mm) One mile equals 1,609,344 millimetres. Dividing by the circumference gives the number of complete rotations per mile. For 205/55R16: 1,609,344 / 1,985 = approximately 811 revolutions per mile. This figure is important for speedometer calibration -- if you change tyre size and the revolutions per mile change significantly, your speedometer will read incorrectly. **Speed Rating:** The speed rating is a simple lookup: S = 180 km/h, T = 190 km/h, H = 210 km/h, V = 240 km/h, W = 270 km/h, Y = 300 km/h. UK law requires tyres to be rated at or above the vehicle's maximum speed.