Decoding the Tyre Sidewall

posted on 2 October 2008

The markings on a tyre may look confusing but they are actually very important. Each marking provides a vital piece of information about the specification of each tyre. This includes information such as the maximum the tyre can safely sustain; the loads it can carry and the safety standards it meets.

What does the writing on my tyre sidewall mean?

In short, the numbers and letters correspond to the exact size and specification of the tyre. This includes details of tyres construction, the speed rating and load index. The easiest way to explain the parts is by using a diagram

1. Width of the tyre in millimetres.
2. Aspect ratio of the tyre. The height of the tyre sidewall as a percentage of the width. In this case it would be XX% of XXXmm.
3. The tyre construction type. The letters will vary dependant on construction.
   4. Diameter of the tyre’s inner rim in inches.
4. The load capacity (see below for more information)
5. Speed symbol (see below for more information)
6. European approval mark indicating the tyre quality has been tested and approved by the European Regulatory Authorities. It is now illegal to sell part worn tyres without an ‘e’ mark
7. “DOT” Code – indicates that tyres are compliant with the US Department of Transportation code.
8. Age symbol denoting date of manufacture. It is recommended that tyres over 6 years are replaced as rubber degrades over time.
9. Denotes direction of tyre rotation when fitted.
10. Indicates tyres are suitable for mud and snow (appears on applicable tyres only)

Understanding Load and Speed Indices

All tyres carry coded markings on them which correspond to their load carrying and speed capabilities. Decoding these markings is simple – when you know how.

The Speed Rating

All tyres are marked with a speed rating letter that corresponds to the maximum speed a vehicle can travel under conditions specified by the tyre manufacturer.
Manufacturers calculate the speed indexes when tyres are in good condition. So these ratings do not apply to tyres which are damaged, under or overinflated, overloaded, repaired or have cuts and bulges.

And remember, just because you may have tyres which can perform at high speeds, no manufacturer would recommend driving above the legal speed limit.

The speed rating table:

What is the Load Index?

The load index is a numerical code which represents the maximum weight which a tyre can safely carry.
Sometimes, tyres will display two load indices, for example 170/160. This means the first number (in this case 170) is the code for a single wheel while the second number (160 in the example) is the code for a twin wheel fitment.
The load index table:

Click here to view the Speed Rating Window in a pop window

Calculating the load and speed ratings

Calculating the load and speed ratings of your tyres is easy:
1. Find the speed and load markings (sometimes referred to as the service description) on your tyre. It is normally adjacent to the tyre size markings
2. Compare the numerical code to the Load Index table below to determine the maximum load in kilograms
3. Compare the letter to the Speed Rating table below to determine the maximum speed capabilities of your tyre.

When fitting new tyres to your vehicle make sure you follow vehicle manufacturers recommended load and speed ratings. Fitting tyres with ratings lower than recommended is not advised and can void your insurance.
If you can’t find the recommended load index but you know your vehicle’s weight you can still calculate the load rating. Let’s assume your vehicle weighs 5000kg and weight is evenly distributed, divide the vehicles weight by 4 to get the load per tyre. In this instance it would be 1250kg which corresponds to a load index of 115. For safety’s sake, it’s always a good idea to add at least 20% to allow for full passengers and luggage. As a general rule, cars have a much higher load rating than will ever be needed.

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