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HOW TO READ A TYRE

On the sidewall of a tyre you will find various codes and markings. The list below aims to simplify those codes and allow you to understand the information imprinted on the sidewall.

Tyre sizes are made up of a number of different numbers and letters. For example the tyre size 255/55R 16 85 V is made up of the following information.

225 This is the tyre width in millimeters.

55 The aspect ratio as a % (the height of the sidewall divided by the tyre's width).

R Denotes the tyre's construction type - in this case it's a radial.

16 This is the rim diameter in inches.

85 This is the load rating of the tyre.

V This is the speed rating of the tyre.

Tyre Age - Tyres carry a three digit age code on the sidewall indicating the month and year of manufacture. For example 129 means the tyre was manufactured in December 1999. Tyres 6 yrs or older need to be changed due to the deterioration of the tyre.

 

OTHER MARKINGS

M&S - Identifies Mud and Snow tyres.

DOT Codes - The requirements of the US Department of Transportation contain a mixture of letters and numbers such as DOT DVDE MTA 129.

E-Marks - Tyres sold in the European Community must carry an E -Mark in accordance with ECE Reg 30 - E.g. E4 027550.

 

ASPECT RATIOS

The Aspect ratio is the ratio of a tyre's width to it's height. So a 55 series tyre is a tyre whose height is equal to 55% of its width.

 

LOAD INDEX

Kg Li Kg Li Kg Li

65

290 85 515 105 925
66 300 86 530 106 950
67 307 87 545 107 975
68 315 88 560 108 1000
69 325 89 580 109 1030
70 335 90 600 110 1060
71 345 91 615 111 1090
72 355 92 630 112 1120
73 365 93 650 113 1150
74 375 94 670 114 1180
75 387 95 690 115 1215
76 400 96 710 116 1250
77 412 97 730 117 1285
78 425 98 750 118 1320
79 437 99 775 119 1360
80 450 100 800    
81 462 101 825    
82 475 102 850    
83 487 103 875    
84 500 104 900    

The load-index figure imprinted on the sidewall of the tyre, normally just before the speed rating letter, denotes the maximum load capacity of a tyre when driven at maximum speed. A list of load indices and maximum weights is give above.

 

SPEED RATINGS

Speed symbol Max speed (km/h) Max speed (mph)
N 140 87
P 150 93
Q 160 99
R 170 106
S 180 112
T 190 118
H 210 130
V 240 149
W 270 168
Y 300 186
ZR 240 149

All tyres carry a speed symbol in the form of a letter indicating the maximum speed for which the tyre is intended. The table below shows which speed goes with each letter.

 

UP STEPPING

Up Stepping to High Performance
Maximizing your car's performance by selecting a lower profile tyre/larger size rim combination is referred to as "up-stepping." Plus-one or plus-two tells you how much larger the rim size is. This "up-step" approach to improving performance came into being because, although it is possible to get higher performance without changing rims, it is often necessary to increase tyre width to the point that steering geometry is affected, with the result that a change in driving style or habits becomes necessary.



So in order to fully maximise your vehicle's performance (which includes preserving the steering geometry) the "up-step" approach is highly recommended.

 

ROTATION

Tyre wear rates differ depending on the axle on which the tyres are fitted and whether the vehicle is front, rear or four wheel drive. To extend the life of your tyres it is advisable to change tyre positions on a regular basis. Different tyre manufacturers may recommend different rotation periods, ranging from 4-10,000Km. It is advisable to switch tyres from left to right periodically as well as from the back to the front of the vehicle.
Certain high performance tyres are position specific and should not be rotated.

 

TREAD DEPTH

Current tread depth legislation requires that car tyres must have a minimum of 1.6mm of tread in a continuous band throughout the central ¾ of the tread width and over the whole circumference of the tyre. To help drivers recognise when their tyres are nearing the legal limit, tyres are manufactured with tread wear indicators in the grooves.

However, it is universally recognised in the tyre industry that the legal limit is wholly insufficient to protect drivers in adverse driving conditions. Drivers are therefore recommended to consider replacing their tyres when the tread depth reaches 3mm.


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