Tyre Markings

WHAT ALL THE NUMBERS MEAN

Each section of the small print on a tyre’s sidewall means something. Let’s say, for example a tyre has the following information on its sidewalls: P235/75 R15; Treadwear 480 Traction AA Temperature A (this is uniform tyre quality grading); 105 S (service description); plus of course the manufacturer’s name and the tyre name. Let us find out what all this information means.

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.

 
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 build-up of heat. The temperature grade applies to a properly inflated tyre that is not overloaded. Under inflation, overloading or excessive speed can lead to more heat build-up. Excessive heat build-up 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. (See chart below)

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, S 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.

 

Maximum Load-Carrying Capacity Per Tyre
Load Index Pounds Kilograms 71 761 345 72 783 355 73 805 365 74 827 375 75 853 387 76 882 400 77 908 412 78 937 425 79 963 437 80 992 450 81 1019 462 82 1047 475 83 1074 487 84 1102 500 85 1135 515 86 1168 530 87 1201 545 88 1235 560 89 1279 580 90 1323 600 91 1356 615 92 1389 630 93 1433 650 94 1477 670 95 1521 690 96 1565 710 97 1609 730 98 1653 750		Load Index Pounds Kilograms 99 1709 775 100 1764 800 101 1819 825 102 1874 850 103 1929 875 104 1984 900 105 2039 925 106 2094 950 107 2149 975 108 2205 1000 109 2271 1030 110 2337 1060 111 2409 1095 112 2484 1129 113 2561 1164 114 2640 1200 115 2721 1237 116 2806 1275 117 2892 1315 118 2982 1355 119 3074 1397 120 3169 1440 121 3267 1485 122 3368 1531 123 3472 1578 124 3580 1627 125 3690 1677

 

 Table showing Speed Ratings for tyres

SYMBOL GUIDE (NORMAL USE)
SPEED SYMBOL	SPEED (kph)	SPEED (mph)
CAR
S	180	111.8
T	190	118.1
U	200	124.3
H	210	130.5
V	240	149.1
W	270	167.8
Y	300	186.4
VR*	210+	130+
ZR*	240+	150+
WINTER/REINFORCED
Q	160	99.4
R	170	105.6
VAN
N	140	87
P	150	93.2
TRUCK
J	100	62.1
K	110	68.4
L	120	74.6
M	130	80.8
*Old designation

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