The wall of your tire is covered in numbers and letters, and most drivers ignore them until it is time to buy a replacement. Learning to read them takes five minutes and it saves you from buying the wrong size or an old tire that has been sitting in a warehouse for years.

The main size code (for example 225/45R17 91V)

This single string tells you almost everything:

  • 225 is the tread width in millimetres.
  • 45 is the aspect ratio, so the sidewall height is 45 percent of the width. A lower number means a shorter, sportier sidewall.
  • R means radial construction, which is what almost every modern car uses.
  • 17 is the wheel diameter in inches.
  • 91 is the load index, and V is the speed rating.

Load index and speed rating

The load index is a code, not a weight in pounds. A 91 supports about 615 kg per tire. Never fit a tire with a lower load index than the factory tire, especially on trucks and vans that carry weight. The speed rating letter (Q, S, H, V, W and so on) is the maximum sustained speed the tire is built for. Match or exceed the original rating.

The date code tells you how old the tire is

Find the four digit code at the end of the DOT stamp. The first two digits are the week and the last two are the year. A code of 3122 means the 31st week of 2022. Rubber ages even when a tire is unused, so avoid buying tires that are more than a couple of years old, and think about replacing any tire that is six years or older regardless of tread.

Frequently asked questions

Can I fit a wider tire than standard?

A small step up can work if it clears the bodywork and keeps a similar overall diameter, but it changes your speedometer reading and handling. Stay close to the factory size unless you know exactly what you are doing.

Does a higher speed rating ride harder?

Often yes. Higher rated tires usually have stiffer sidewalls, which can feel firmer over bumps but sharper in corners.

Where is the date code on my tire?

It is on the sidewall as part of the DOT code. On some tires the full code is only on the inner sidewall, so you may need to look on the back.

Once you can read the sidewall, choosing a replacement is simple. Browse our tire buying guides to compare options that match your size and driving.

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Video: Related tutorial from YouTube