How do I find out my tyre size, and what size tyres will fit my wheels?
We’ve been talking about alloy wheel repair, refurbishment and their general upkeep recently, so a topic that links nicely with those is how we determine which tyres are the correct option for your wheels.
It’s fair to say ‘how to read tyre size’ is one of the regular questions we get asked by customers trying to find the best tyre choice and subsequent prices for their car.
Tyre size explained
In this article, we’re going to provide a simple tyre guide to explain the numbers and letters around the wall of each tyre. This should help you choose the right ones for your wheels and your car. By the end of this article, you’ll understand how to identify tyre size and what the fundamental characters in the system mean.
How to find my tyre size
It’s a relatively simple system. The numbers and letters are easy to find; they should be the most noticeable thing on the wall of your tyre apart from perhaps the brand name or logo.
Let’s take a look at an example tyre size, the numbers and letters, and dig into what they mean:
Example tyre size: 195 / 55 R16 96W
What the numbers mean: Width / Profile / Size / Speed
These are the basic elements, and for most drivers, it’s all you need to know. In fact, most drivers won’t even need to know these details. If you need new tyres, it’s more than likely that your mechanic or tyre depot will take care of all of that for you. It’s their job, after all. If you get back on the road with the minimum of fuss, then why do you need to know?
Well, if you’re thinking of changing wheels, or you have already, then you’ll need to know what tyres you can run and if the new combination will fit safely onto your car.
The first number in the sequence outlines your overall tyre width. This is a simple measurement in millimetres. So, in our example above, the tyre is 195mm from one side to the other.
On typical passenger car tyres, there may be a letter P in front of the sequence. This stands for Passenger and is only shown on tyres that conform to P-metric options. For more industrial and heavier use tyres, this could be replaced by LT, which stands for light-truck. Other letters you might see are T, for temporary spares, or ST, for special trailer.
Tyre profile or aspect ratio
The second number, in our case 55, is the profile of your car tyre. The profile is the height of the tyre sidewall as a percentage of the width. So, the 55 we see means that the tyre height is 55% of the width (195mm). A simple calculation gives us a figure of 107.25mm.
Tyre size, or diameter
The third number, preceded by an R (the R indicates a Radial construction), is the diameter of the tyre’s inner rim measured in inches.
Some tyres will have a D or B where the R appears. These letters indicate a diagonal or bias ply construction. However, almost all tyres are manufactured with radial construction these days.
Tyre loads and speed ratings
The final set of characters indicates the load index and the speed rating.
In our example, the 96 dictates the load index. Load index defines the maximum weight a car should carry when driven at full speed. The index ranges from an index of 70 with a maximum load of 335kg to an index of 775 with a maximum load of 1060kg.
Our 96 load index tells us that the maximum load for our tyres is 710kg.
Speed ratings represent the maximum speed the tyres should travel with a full load; so kind of the opposite of the load indexes. Shown as a letter, they don’t quite conform to a simple, easy-to-understand order.
When you consider some of the most common speed ratings, for example, maximum speeds that run from 100mph to 186mph, you’d expect them to run in alphabetic order—but no. Take a look:
- Q – max speeds up to 100mph (160km/h)
- R – max speeds up to 105mph (170km/h)
- S – max speeds up to 113mph (180km/h)
- T – max speeds up to 118mph (190km/h)
- H – max speeds up to 130mph (210km/h)
- V – max speeds up to 150mph (240km/h)
- Z – speeds over 150mph (240km/h)
- W – max speeds up to 168mph (270km/h)
- Y – max speeds up to 186mph (300km/h)
Additional symbols, numbers and letters
As well as the sizes and symbols already mentioned, there are a host of additional details given on the wall of your tyre.
Reinforced tyres: Where you see REF on a tyre, this indicates that it has been manufactured to handle extra weight. This can vary by manufacturer so keep an eye out for XL and EXL (extra load) and RF, REINF and RFD (reinforced).
Date of manufacture: The letters DOT indicate further details required by Department of Transport safety standards. The last 4 digits in this sequence identifies the week and year of manufacture. So, 2318 would mean the 23rd week of 2018. Tyres degrade over time, especially when they’re not used regularly. If your tyres are over 6-years-old, we’d suggest checking that they’re still in the correct condition for use.
Run flat tyres: A safety feature integrated into some of the tyres on today’s market means that they can run for a limited time after a puncture or losing pressure. The walls are reinforced to be strong enough to support the vehicle weight for a short time—ideally long enough to get to safety. Keep an eye out for ROF, EMT, RFT, ZP, SSR or DSST. They all mean the same thing but are the different indicators chosen by different manufacturers.
Winter and severe condition tyres: The last marking we’re going to mention is the M+S (mud and snow) indicator. There’s also a 3-peaked mountain logo with a snowflake that dictates the same thing. As you’d imagine, these tyres are created to provide added grip in severe conditions or for dedicated winter use.
What size are my tyres?
Now you’ve got all the information you should be able to determine your tyre size for yourself. There are plenty of calculators online that will tell you the correct tyre for your car, as long as the wheels are the same as they were at manufacture. They’ll help you find the recommended option.
But what if you want to change your wheels?
We’ve been discussing the advantages of replacing steel or an old set of wheels with a set of alloys. Our alloy wheel measurement guide has shown the elements to consider when making a replacement so they perform as they should, and don’t cause problems when driving.
Can you change tyre size? Well, if you’re putting a bigger wheel on your car, then you must. To retain the correct readings for mileage, speed and fuel economy, your new wheel and tyre combination should match the same outside running circumference (the distance your car travels on one full revolution of your wheels).
Low profile tyres for alloy wheels
In our next article, we’re going to dig deeper into what it means to run low profile tyres with your wheels; the elements to consider with a tyre conversion for bigger wheels, and we’ll walk you through all of the pros and cons.