What Do The Numbers On Tyres Mean?
Tyres. The black things on your wheels that help you stick to the road. But have a closer look and you’ll see that there’s a bunch of numbers written on the side of those tyres. In fact, there’s a lot of info on the sides of tyres that most people don’t even glance at, because you have to decipher it. One of these codes is tyre size, so when we ask “what size is your tyre?” then “they’re 18’s” just isn’t enough. Here’s why:
Tyres sometimes start with one or more letters, such as P or LT, but a lot of tyres don’t have any letters at all. The letter explains what the tyres were designed for in terms of the vehicle or the service, so what if there’s no letters? Let’s break it down even more.
P = P-Metric
For example, P225/55R17 88T.
These are the most common type of tyres. The P stands for Passenger Vehicles, so the tyres are designed for use on vehicles like cars, minivans, light duty pick-ups etc.
LT = Light Truck
For example, LT235/75R15 104-101S/C
Light Truck Metric tyres begin with the letters LT. These are the tyres designs for use on vehicles that carry heavy loads or tow other vehicles, or a whole range of types of pick up trucks.
For Example 225/55R17 88T
That example looks familiar right? It’s the same code as the P example, but without the “P”. That’s because they’re largely the same tyre - suppliers from Europe don’t include the letters, suppliers from the US do include them. Chances are if you’re in the UK then your garage will use Metric tyres, so you’ll be most familiar with this type.
Tyre Sizes Explained
Okay, lets start breaking down the rest of the code. We’ve got the section width of the tyre, the aspect ratio of the sidewall, the wheel diameter and the speed rating.
Section Width of the tyre
This number is the 3 digits that either come after the letter, or are just the start of the code. For example 225/55R17 indicates that the tyres are 225mm wide from the inner sidewall to the outer sidewall, when properly mounted on a wheel. If you want to find the size in inches (for the few americans that may find this post, divide the number by 25.4)
Sidewall Aspect ratio
Going back to the example of 225/55R17, the 2 digit number following the forward slash (the /) is the sidewall aspect ratio, also called the “tyre Profile”. In our example, the 55 indicates that the distance between the rim of the alloy to the outer part of the treat is 55% of the section width, or 55% of 225. A lower number means a lower profile, or a shorter sidewall. These can be found on newer, performance cars like BMWs or Audis. A larger number means a more chunky looking wheel, which are usually found on your more basic cars, particularly learner friendly cars.
What’s best though? Click to see the difference between low profile and high profile tyres.
This is the letter. R indicates a Radial construction. B means Belted Bias construction and D is for Diagonal Bias. Most tyres you’ll find will be R’s.
Tyre and Wheel diameter
Again using 225/55/R17 as our example, the 17 indicates that these tyres are to fit a 17” diameter wheel. tyres usually come in the following widths (in inches) 14, 15, 17, 18, 19, 20, 22, 23, 24 and 26, though you can get bigger and smaller sizes.
Unique wheel diameters, such as 14.5, 15.5 etc exist, but are less common. These tyres are used for vehicles carrying higher loads. Our only point here is that make sure you check the tyres with the wheels sizes. They need to match, otherwise mounting the tyres will be difficult and if you do manage to do it, driving on them will be dangerous.
Service Description Rating
Or the Load Index and Speed Rating. For example, 225/55R17 91V gives us 91V as our service description. 91 is the load index, or how much weight the tyres can handle, and V as the speed rating. The speed rating is the maximum speed and is given as a letter, and as a guide:
Q = 100mph
R = 112mph
S = 118mph
T = 124mph
U = 130mph
V = 149mph
W = 168mph
So, now you should be able to decipher the numbers on your tyre! Remember that the other important number for your tyres is 1.6. That’s 1.6mm, the legal limit for tread depth. Have a feel on the inner side and the outer side of the grooves in your tyre and feel for the bump. If your tyres tread is higher than the bump, you’ve still got some distance to go. If the bump matches the rest of the tyre, you need new ones! While you’re here, why not just press the “book now” button on the homepage and get booked in for new tyres with Brendon Garage.
Stay safe this winter, if you have any concerns then book a visit in, or drop by:
Special thanks to Matti from MWM Digital for helping us with the blog!