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How To Read A Tyre: Understanding Tyre Size, Load Index, And Speed Rating

Every tyre sold by Tyre Shopper has markings on the tyre sidewall to indicate key information. This system of markings indicates the tyre size, tyre speed rating, model, and load index. Both low-profile and regular tyres use this marking system as standard, so it’s important that you know how to read and understand tyre markings.

According to TyreSafe, there has been 5,375 total casualties from tyre-related car accidents in the last five years and 10.4% of failed car MOTs in the UK in 2017 were caused by tyres and wheels. Make sure you know how to buy the right tyres and check their key information with this guide.

How to read tyre size

With a little reading, it's easy to understand what the series of letters and numbers mean on your tyre's sidewall. Taking the image above as an example, we can work out vital information about the tyre that you will require if you ever need to replace it.

If the tyre reads 255/40 R20 101V:

255 Refers to the width of the tyre, measured in mm
40 Is the height/profile aspect ratio as a percentage of the width of the tyre
R20 Indicates the diameter of the rim, measured in inches
101 Shows the Load index per tyre (see next table for corresponding weight)
V Speed rating code (see table for corresponding speed)

What is the tyre load index rating?

Some vehicles are able to carry greater loads and therefore need a higher inflated tyre pressure reading. Tyres designed for higher loads bear the markings XL (Extra Load) or RF (Reinforced).

The tyre load index is the maximum capacity that each individual tyre can carry. It is usually represented in kilograms.

LOAD
INDEX

KG MAX. LOAD
PER TYRE
LOAD
INDEX

KG MAX. LOAD
PER TYRE
LOAD
INDEX

KG MAX. LOAD
PER TYRE
62 265 84 500 106 950
63 272 85 515 107 975
64 280 86 530 108 1000
65 290 87 545 109 1030
66 300 88 560 110 1060
67 307 89 580 111 1090
68 315 90 600 112 1120
69 325 91 615 113 1150
70 335 92 630 114 1180
71 345 93 650 115 1215
72 355 94 670 116 1250
73 365 95 690 117 1285
74 387 97 730 119 1360
76 400 98 750 120 1400
77 412 99 775 121 1450
78 425 100 800 122 1500
79 437 101 825 123 1550
80 450 102 850 124 1600
81 462 103 875 125 1650
82 475 104 900 126 1700
82 487 105 925    

What does tyre speed rating mean?

Speed rating refers to the maximum speed a tyre can sustain under its recommended load capacity. These ratings are based on extensive testing carried out by the vehicle manufacturer, and the speed rating should not be exceeded.

Generally, the higher the tyre speed rating, the better quality the tyre. Being aware of your tyres' speed rating allows you to determine the maximum speed your tyre is capable of safely maintaining and is important when deciding upon new tyres. If you choose a tyre with a lower speed rating than is recommended, your insurance policy may be invalidated in case of an accident.

SPEED
RATING

KM/H

MPH
SPEED
RATING

KM/H

MPH
A1 5 3 K 110 68
A2 10 6 L 120 75
A3 15 9 M 130 81
A4 20 12 N 140 87
A5 25 16 P 150 87
A6 30 19 Q 160 100
A7 35 22 R 170 106
A8 40 25 S 180 112
B 50 31 T 190 118
C 60 37 U 200 124
D 65 40 H 210 130
E 70 43 V 240 149
F 80 50 W 270 168
G 90 56 Y 300 186
J 100 62 (Y) 300+ 186+

Tyre sidewall heights

Low profile tyres have a slightly different marking system to regular tyres. After the tyre section width figure, you'll find a number which indicates the sidewall height.

For example, in 255/40 R20 101V, the 40 means that the sidewall height is 40% of the tyre section width.

With a standard tyre, the sidewall height is usually 82% of the nominal section width.

Can I use a different tyre size?

Drivers should use the tyre size that is specified for the wheels on their car, with the profile and width that is specified in the car’s manual. However, a change in tyre size is possible when bearing in mind the wheel offset/inset value. This will help to avoid over-sized wheels and tyres becoming a hindrance to the functioning of the wheel arch or vehicle suspension.

When changing a tyre size, you should ensure that the tyre and wheel assembly diameter of the same value so that the speedometer and gearing remain unchanged. For each 10mm boost to tyre width, a reduction of 5% in the height of the sidewall is necessary, if the wheel size is unchanged. Of course, the tyres and wheels that you have selected must fit under the wheel arch without making contact with it or disturbing the vehicle’s suspension.

Our expert guidance for tyre sizes

Tyre Shopper can provide expert advice to buyers looking for tyres online, whether to change their tyre size or wheel size, so contact us today. We have 230 fitting centres across the country.

Please note that such changes are considered a modification by some UK insurance companies.

Back to Tyre Information

You may also be interested in the following article:

Tyre Sizes Explained provides you with further information to help you understand which tyre size you will need for your vehicle.

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