What do the numbers on a tyre sidewall mean?

Sidewall markings look like a code because they are one, but there are only six pieces of information in the main string and each one is a simple physical fact about the tyre.

Step by step

1. Read the first number as width in millimetres.205 means the tread is nominally 205mm across. Wider is not automatically better — the car was designed and tested around a particular width.
2. Read the second number as a percentage, not a measurement.55 means the sidewall height is 55 per cent of the width. A lower number means a shorter, stiffer sidewall: sharper steering, harsher ride, more vulnerable to potholes.
3. Take R as construction.R is radial, which is what virtually every modern car tyre is. RF or a similar marking, or a maker-specific code such as ZP or RunOnFlat, indicates a run-flat, which must not be mixed with conventional tyres.
4. Read the next number as rim diameter in inches.16 means it fits a 16 inch wheel. This one is absolute — a tyre of the wrong rim size simply cannot be fitted.
5. Look up the load index.91 is a coded figure, not a weight in kilograms. Your handbook or the door placard states the minimum index for your car, and a replacement must meet or exceed it.
6. Read the letter as a speed rating.It is the maximum speed the tyre is constructed for. Fitting a lower rating than the car specifies is not permitted and can affect insurance, even though you will never drive at that speed.
7. Find the DOT date code.A four-digit number, usually in an oval, giving the week and the year: 2321 means the twenty-third week of 2021. Worth checking on a used car, and on a spare that has never been used.
8. Note the extra letters that matter.M+S and the three-peak snowflake mark winter capability. XL or Reinforced means a higher load capacity. A letter and number such as MO, AO or * is a manufacturer-approved fitment for particular cars.
9. Match the whole set when replacing.Same width, profile and rim size on an axle, and load and speed ratings that meet the car's specification. Mixing sizes across an axle is a straightforward MOT failure.

Tips

• The placard in the door shut states the sizes and ratings your car was approved with. Photograph it once and you never have to guess.
• A shorter sidewall looks better and rides worse. Changing profile to fit a bigger wheel has real consequences on a British road surface.
• Never mix run-flats and conventional tyres on the same car unless the manufacturer explicitly allows it.

Common mistakes

• Treating the load index as a weight in kilograms — It is an index into a standard table. A difference of a few points is a substantial difference in capacity, and fitting below the specified index can overload the tyre.
• Buying on price alone across an axle — Two different tyres on the same axle behave differently under braking and in the wet. Matching pairs is more important than the brand name on them.

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