Common tyre sizes

Tyres, those round, black rubber things on the four corners of our vehicles are something that most people take for granted. Until, that is, we have a puncture or need to buy replacements when they are worn out.

That’s when we get into the mystifying world of tyre sizes - all of those letters and numbers that are written around the tyre’s sidewall. When you know how to unpick the code it’s really not that mysterious at all and it can help you to go shopping for new tyres with confidence because you understand exactly what you need.

Understanding tyres

Next time you’re cleaning your car or checking the tyre pressures (you do check the pressures?) have a look at the information that is printed on the side of the tyre. There will be the maker’s name, such as; Pirelli, Falken, or Sumitomo and maybe a model name as well. There will probably be an arrow showing which way round the tyre should be fitted. There will also be a string of letters and numbers.

This will be something along the lines of 205/55 R16 - this is one of the most common tyre sizes in the Nigeria but more of that later - followed by a number and a letter such as 91V. If you know how to interpret this information, it will tell you everything you want to know about the tyre and what to look for to get a replacement.

Okay, so let’s unpick the code. The first part of the number, 205, tells you what the width of the tyre is. This is given in millimetres, so a 205 tyre will be 205mm wide across the tread. The next two numbers immediately following the slash define the aspect ratio. This ratio describes the height of the sidewall, not in millimetres this time, it’s given as a percentage of the width. Therefore, 55% on our 205 tyre means it has a side wall height of 113mm. If you hear people referring to ‘low profile’ tyres, they are talking about the aspect ratio, the smaller the figure you have for this the lower the profile of the tyre.

Next comes a letter. In the majority of cases, this will be an ‘R’. The R tells us that this is a radial tyre. This means that the cords that give the tyre its strength run radially around they tyre at 90 degrees to the tread. All tyres on modern cars are radials because they grip, ride and handle better. However, on some older classic models you can still come across cross-ply tyres, these will be marked with a letter C.

Now we have another two-digit number, this is the size of the wheel that the tyre will fit. Just to mix up the measurements a little more, this is expressed in inches, so the 16 in our example means the car has 16-inch diameter wheels.

The wheel size when combined with the aspect ratio of the tyre gives a total rolling diameter. It’s important to understand this because car makers will often put different wheel and tyre combinations on different models. For example, the most basic model in a range might have 15-inch wheels fitted with 65 aspect tyres, but a higher spec version of the same car will have 16-inch wheels fitted with a 55-aspect tyre.

The key thing to note here is that the total rolling diameter - the wheel and tyre together - will be the same. If you are considering upgrading your car by fitting alloy wheels, therefore, it’s essential to get both the wheel and tyre size correct. Failure to do so will risk your speedo misreading and could also mean tyres rubbing on the bodywork. The same applies if you are buying a spare wheel for a car that came with a puncture repair kit; you don’t necessarily have to have the same size as the as the road wheels provided that the rolling diameter matches.

Which brings us to the final marking on the side of the tyre. This will be another letter, often V or W, which refers to the tyre’s speed rating, followed by a number such as 91 which indicates the load that the tyre can carry. When replacing tyres, you should, where possible, try to match the original rating. Fitting a tyre with a higher speed or load rating won’t do any harm, but absolutely avoid getting a lower one.

Popular tyre sizes & prices

Now we know what sort of tyres we have, we can see if they are one of the most common tyre sizes or something a little more unusual. The ten most common sizes in the Nigeria are:

Why are these the most common tyre sizes? Because they are fitted to the most popular vehicles. If you drive a model such as a Ford Mondeo, Audi A3, Honda Civic or BMW 3 Series, there’s a good chance you will have a tyre, such as; a 225/40 R18. On a large 4x4 such as a Range Rover Sport or Mercedes GLE, you are more likely to find a bigger, wider tyre, such as; a 275/40 R20. On a small hatchback such as a Ford Fiesta or Kia Rio, you’ll find something like a 195/55 R16.

The important thing is to check YOUR car because specifications can vary over various releases of a model and between trim levels, or a previous owner may have fitted some different wheels and tyres at some point. Check the tyres themselves or look in the car’s owner’s manual if you are unsure.

It’s also important to note that even when you’ve understood the size, there are still different types of tyre. There are winter tyres to give better grip in bad weather, for example. Some tyres offer a lower rolling resistance in order to save fuel and some vehicles need specialist run-flat tyres.