HOW TYRE SIZES WORK – WHAT DO THE NUMBERS MEAN?Date: 2018-01-17
It’s that dreaded time during your car’s life that it needs new rubber! Dreaded because, these days, replacing a full set of four tyres can easily set you back R6 000 to R10 000 for a mid-size sedan or compact SUV.
If you want to perfect the ultimate look for your ride you will have to know these sizes well. Well, this is the reason for this handy little guide.
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 – so here’s a quick rundown to help you out:
A – Width (mm)
The width of the tyre, from sidewall to sidewall, in millimetres. In this example, the 195 means the tyre 195 millimetres wide. The tread width is actually somewhat smaller than the tyre’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 tyre’s firmness increases. Smaller aspect ratios, also called ‘low profile’ tyres are generally reserved for performance tyres on sports cars.
C – Rim Size (Inches)
The tyre’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.
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 tyres are capable of are shown below:
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. Tyres with higher speed ratings are constructed to handle heat better, but with the compromise that they also ride harder than tyres 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 and, because of this, they don't conform to conventional passenger vehicle nomenclature. In this case they read like so: Michelin Pilot SportPAX 245/690 R520 tyres in the front and 365/710 R540 at the rear. The 690 and 710 in these respective codes refer to the tyre's overall diameter and not the profile. The actual profiles? 35 up front and a paltry 25 at the rear. Oh yes, and they cost about R140k each!
Now, the R8 000 you have to spend for all four of your Golf GTI tyres doesn’t look too bad, does it?