WHAT ALL THE NUMBERS MEAN

Tyre Type
The P designates that the tyre is a passenger vehicle tyre. Some other designations are LT for light truck, and T for temporary, or spare tyre.

Tyre Width
The 235 is the width of the tyre in millimetres (mm), measured from sidewall to sidewall. Since this measure is affected by the width of the rim, the measurement is for the tyre when it is on its intended rim size.

Aspect Ratio
This number tells you the height of the tyre, from the bead to the top of the tread. This is described as a percentage of the tyre width. In our example, the aspect ration is 75, so the tyre’s height is 75 per cent of its width, or 176.25mm (75 x 235 = 176.25mm, or 6.94in). The smaller the aspect ratio, the wider the tyre in relation to its height.
High performance tyres usually have a lower aspect ratio than other tyres. This is because tyres with a lower aspect ratio provide better lateral stability. When a car goes around a turn lateral forces are generated and the tyre must resist these forces. Tyres with a lower profile have shorter, stiffer sidewalls so they resist cornering forces better.

Tyre Construction
The R designates that the tyre was made using radial construction. This is the most common type of tyre construction. Older tyres were made using diagonal bias (D) or bias belted (B) construction. A separate note indicates how many plies make up the sidewall of the tyre and the tread.

Rim Diameter
This number specifies, in inches, the wheel rim diameter the tyre is designed for.
In America, passenger car tyres also have a grade on them as part of the uniform tyre quality grading (UTQG) system. The tyre’s UTQG tells you three things:

Tread Wear: This number comes from testing the tyre in controlled conditions on a government test track. The higher the number, the longer you can expect the tread to last. Since no one will drive his or her car on exactly the same surfaces and at the same speeds as the government test track, the number is not an accurate indicator of how long the tread will actually last. It’s a good relative measure, however: You can expect a tyre with a larger number to last longer than one with a smaller number.

Traction: Tyre traction is rated AA, A, B or C, with AA at the top of the scale. This rating is based on the tyre’s ability to stop a car on wet concrete and asphalt. It does not indicate the tyre’s cornering ability.

Temperature: The tyre temperature ratings are A, B or C. The rating is a measure of how well the tyre dissipates heat and how well it handles the buildup of heat. The temperature grade applies to a properly inflated tyre that is not overloaded. Underinflation, overloading or excessive speed can lead to more heat buildup. Excessive heat buildup can cause tyres to wear out faster, or could even lead to tyre failure.

Service Description
The service description consists of two things:

Load Ratings: The load rating is a number that co-relates to the maximum rated load for that tyre. A higher number indicates that the tyre has a higher load capacity. The rating ‘105’, for example, corresponds to a load capacity of 2039 pounds (924.87kg). See the chart for all the ratings. A separate note on the tyre indicates the load rating at a given inflation pressure.

Speed Rating: The letter that follows the load rating indicates the maximum speed allowed for this tyre (as long as the weight is at or below the rated load). For instance, 5 indicates that the tyre can handle speeds up to 112mph (180.246kmph). See the chart for all the ratings.

Calculating the Tyre Diameter: Now that we know what these numbers mean, we can calculate the overall diameter of a tyre. We multiply the tyre width by the aspect ratio to get the height of the tyre.

Tyre height = 235 x 75 per cent = 176.25mm (6.94in)

Then we add twice the tyre height to the rim diameter.

2 x 6.94in + 15 inches = 28.9in (733.8mm)

This is the unloaded diameter; as soon as any weight is put on the tyre, the diameter will decrease.

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