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How tyre sizes work – What do the numbers mean?

August 15, 2017

It’s that dreaded time during your car’s lifetime that it needs new rubber! Dreaded, because to replace a full set of four tyres (unless you drive a Reliant Robin!) can easily set you back R6000 to R10000 for a mid-size sedan or compact SUV these days.

Memory fails us all and if you are anything like us, each time you need new tyres you need to study the tyre sizes and what those numbers & codes mean from scratch… Well, this is the reason for this handy little guide.

Tyre Sizes numbers Explained
Tyre sizes | What the numbers mean

Together with the manufacturer’s name and the name of the tyre (e.g. Goodyear Eagle), there’s also a set of numbers and letters on the tyre’s sidewall. The numbers are indicators of the size, type, capabilities and performance of the tyre.

With a little explanation, it’s easy to understand what all of the letters and numbers mean – Here’s a quick reference rundown on what those numbers mean:

Tyre Sizes numbers Explained
Tyre sizes | Numbers A, B & C

A – Width (mm)

The width of the tire, from sidewall to sidewall, in millimetres. A 195 is 195 millimetres wide. The tread width is actually somewhat smaller than the tire’s actual width.
Overall width of a tyre is the measurement from side to side including any additional sidewall items such as kerbing ribs, raised letters etc. This is important, particularly when upgrading tyre size, as different manufacturers tyres may have different overall widths even though the tyre size is the same.
Section width is the measurement of the tyre from side to side excluding any additional sidewall items such as kerbing ribs, raised letters etc.

B – The Profile / Aspect Ratio (%)

This is the height of the tyre’s sidewall expressed as a percentage of the tyre’s section width (A). For example: a 195/55 tyre is 55 percent as tall as it is wide hence also referred to as the aspect ratio.
To get the actual profile size in mm of the above example: (195mm X 0.55 = 107mm)
As the ‘aspect ratio’ decrease, the tire’s firmness increases. Smaller aspect ratios, also called ‘low profile’ tyres are generally reserved for performance tires on sports cars.

C – Rim Size (Inches)

The tire’s inner diameter, which matches the outer diameter of your car’s wheels. This is measured in inches and taken from the wheel flange where the tyre is seated to the same point on the opposite side. The ‘R’ prefix refers to the tyre’s construction type and stands for ‘Radial’ (Or radial ply construction). All modern cars and light trucks use radials today.

Tyre Sizes numbers Explained
Tyre sizes | Tyre sizes | Numbers D & E

D – Load Capacity

The load rating is represented by a numerical code and is associated with the maximum load that the tyre can carry when operating at its maximum speed (maximum inflation pressure) up to 210kph.
Load ratings range from 0 to 279 and each has a corresponding weight associated with it.
This can get quite technical, but if you intend to tow a heavy caravan or trailer for example, you will require tyres with a higher load rating. Have your local dealer advise you on the correct rating for your setup.

E – Speed Rating

All passenger car tyres have a speed rating expressed as a letter. Those letters and the corresponding speeds the tires are capable of are shown below:

Speed Rating Kilometres per Hour
N 140
P 150
Q 160
R 170
S 180
T 190
U 200
H 210
V 240
Z 240+
W 270
Y 300

Because higher speeds mean greater heat build-up, the Speed Rating is actually only an indication of a tyre’s ability to dispel heat to avoid a blowout. Tires with higher speed ratings are constructed to handle heat better, but with the compromise that they also ride harder than tires with lower speed ratings.

A tyre marked with a ‘Y’ speed rating, indicates it can be ‘safely’ operated at a maximum speed of 300kph. So, if you drive a Bugatti Veyron or something similar you will probably have these installed.

Incidentally, with the Bugatti Veyron being capable of speeds of over 400km/h, Michelin had to specifically develop tyres to perform at this level. The Michelin Pilot Sport PAX 245/690 R520 tyres in the front and 365/710 R540 rear tyres on the Veyron costs about R140k each! Now, the R8000 you have to spend on your Golf GTI tyres doesn’t look too bad, does it?


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