Have you ever looked at the string of numbers and letters embossed onto the rubber of your tyres and wondered what they all mean? They are actually quite simple once you have the code (which you can jot down on a scrap of paper or save in your phone so you do not have to memorise it) and detail exactly what sort of a tyre you are looking at. Let’s take a look at a mythical tyre reading and analyse what it means.
The tyre code used for the purposes of this exercises is: 185/75R14 89T.
185mm: Tyre Width
The first three-digit number refers to the tyre’s width. These can range from as narrow as 125mm (although 165mm and up is more common) up to 335mm (although some custom vehicles sport tyres that are over 400mm wide). Wider tyres offer a more secure grip on the road, but, all too often, wider tyres are chosen because they look muscular and cool, rather than because the driver actually needs the extra control. It should be noted though, that this effect is reversed on muddy or slushy roads, where narrower tyres are more effective!
75%: Aspect Ratio
The second set of numbers, following the division, always two digits is an expression of a percentage: the aspect ratio of tyre width to give your sidewall height. So, in this case, it is easy enough to work out that 75% (or three-quarters) of 185mm is 139mm: and this is the sidewall height of our exemplar tyre.
R: Details of Tyre Manufacture
The R in this case refers to ‘radial’ as this tyre is designed with radial belt construction. Other construction types that you might find are B (bias belt), D (diagonal) or it could be a cross-ply tyre which is not usually marked and is generally a sign of an older tyre. Radial is the most common type of tyre construction seen today.
14: Diameter of Tyre
This number is a straight-forward measurement in inches of the diameter of the tyre when viewed as a circle. Your tyre diameter should always be as per the operators hand-book as changing the diameter of the tyre affects your odometer and your speedometer, making it seem as though you are, respectively, travelling more or less than the distance you are actually covering, and going faster or slower than your actual speed, depending on whether you buy larger or small diameter tyres. It is recommended that you remain within a narrow 2% range of your original tyre size for legal compliance and the correct functioning of these gauges.
89: Load Index
This number is a hard one to know off by heart as each number refers to a range specific maximum load: in this case, 89 means that the vehicle can carry up to 580kg. The grading is weighted, so there is no simple way of working out what the code means: you will have to look it up each time you see a new code to work out what maximum weight that car can carry.
T: Speed Index
The terminal letter of the code refers to the maximum speed that the tyre can be expected to withstand. In this case, T is 118 miles per hour, which does not seem excitingly fast, but is plenty for the daily commute and for use on the UK’s roads which tend to have an absolute upper limit of 70mph. It is only regular users of Germany’s autobahn and the Isle of Man’s unrestricted road networks who might have the need for a speedier tyre.
Now you know how to break down your tyre’s mysterious coding, you can order a new set of tyres with the confidence of knowing exactly what it is that you are looking at. Enjoy special summer discounts at Dartford Tyres. Visit their website to get your hands on cheap tyres.