How to Read the Numbers on a Car Tyre

The string of numbers on a tyre's side, such as 205/55 R16 91V, describes its width, shape, wheel size, load capacity and speed rating. Each part has a fixed meaning. This guide decodes them one at a time using the examples in two public references, and it explains the four-digit date code that tells you when the tyre was made.

The size code, piece by piece

Goodyear's guide uses the example 205/55 R16 91V. Here is each part:

• 205 is the width in millimetres, measured from one sidewall to the other.
• 55 is the aspect ratio: the sidewall height as a percentage of the width. Here it is 55% of 205 mm.
• R means radial construction. Goodyear notes that almost every new tyre today is radial.
• 16 is the wheel (rim) diameter in inches.
• 91 is the load index.
• V is the speed rating.

You may see a letter in front of the size. Goodyear says a P, or no letter at all, points to a passenger car tyre. Wikipedia's tyre code article, which uses the example P215/65R15 95H, adds that P also shows the load tables follow the Tire and Rim Association standards, and that without an aspect ratio the figure is assumed to be 82%.

Work out the sidewall height

Multiply the width by the aspect ratio. For 205/55, that is 205 × 0.55, about 113 mm. A lower ratio means a shorter sidewall, which is why wide, low-profile tyres look thin from the side.

Load index

The load index is not a weight. It is a code that you look up in a table. Goodyear gives 615 kg as the figure for index 91. Wikipedia's example gives 690 kg for index 95. Each tyre carries its own rated load, and the vehicle's total is shared across four.

Speed rating

The letter after the load index is the highest speed at which the tyre can carry its rated load when correctly inflated. Goodyear gives 240 km/h for V. Wikipedia gives 210 km/h (130 mph) for H. The rating is a design limit, not a recommendation to drive at that speed, and speed limits still apply.

Other markings on the sidewall

Goodyear's page lists several more:

• An ECE mark, showing the tyre meets UNECE standards for pneumatic tyres.
• A maximum inflation pressure, which is the most the tyre can take, not the pressure to use.
• TWI marks, showing where the tread wear indicators sit. The page says the minimum tread depth is 1.6 mm in most EU countries.
• Original-equipment marks such as AO for Audi, MO for Mercedes or a star for BMW, showing the tyre was approved for that maker.

Reading the date code

A tyre's age can matter as much as its tread. Goodyear says the production date is a four-digit code giving the week and year of manufacture. Wikipedia explains the longer DOT code used in the United States: after the letters DOT, the first three characters identify the plant, the next two are a size code set by each maker, and the last four digits give the week and year. Tyres made before 2000 used three digits.

For example, a final group of 2324 would mean week 23 of 2024. This is an illustration of the rule, not a reading of a real tyre.

Grades for wear, grip and heat

Wikipedia also mentions UTQG grades, which are wear, traction and temperature ratings for passenger tyres under the Uniform Tire Quality Grading standard.

Matching a replacement

Goodyear's advice is to match the load index and speed rating to the vehicle manufacturer's recommendations. The recommended size is normally listed in the owner's manual. This article is general information; for fitting and safety decisions, rely on the vehicle manual and a qualified tyre fitter.

Sources

• How to read your tyre sidewall markings, Goodyear
• Tire code, Wikipedia