Words and Photos: Craig Cheetham
Wheels and tyres are a mixture of metric and imperial, but what do those numbers mean? Let’s explain…
Unless you’ve had the pleasure of a Land Rover Defender, the majority of cars built from 1970 onwards will have their key dimensions recorded in millimetres. But even if you don’t own an old Land Rover, there’s one area of every car that’s a confusing mix of metric and imperial almost universally – and it’s one that you might never have given a second thought to before: your wheels and tyres.
This is because the diameter of a wheel is measured in inches, but everything else about it – the profile, width and PCD (which we’ll explain in a moment) is recorded in mm. First, though, a history lesson…
Why is tyre and wheel size a mix of metric and imperial?
Strange, isn’t it? But there is some logic behind this mishmash of measurements. As we moved towards a metric system in the early 1970s, the decision was made by the tyre industry to introduce metric tyre measurements pretty much at the same time as cross-ply tyres gave way to steel-belted radials, which is how most tyres are today.
But while this metric system meant width and tyre height (or profile) were standardised, the issue was that there were millions of cars on the road all over the world with imperial-sized wheels, so tyre makers stuck with these imperial diameters. Because there has never been a natural changeover point, as at any given time there have always been millions of cars on the roads, there imperial diameters have remained – it’s as simple as that.
A few car manufacturers did try to change the game, notably BMW, Jaguar and Austin-Rover, all of whom introduced metric diameter wheels. But all this achieved was to make it very difficult to buy tyres for certain older models today, as the metric wheel never caught on.
What do the numbers on a tyre mean?
The largest number you’ll see on a tyre is its actual size, so to take 205/55/R16 as a popular one what that means is as follows:
205/55
The first of the two numbers is the width of the tyre across its tread. It is shown in millimetres and defines the distance between the inner and outer sidewall of the tyre. For example, 205 means that your tyre is nominally 205 mm wide. The second number is the aspect ratio – or the relationship between a tyre’s sidewall height and the tyre’s width. It is expressed as a percentage. For example, 55 indicates that the sidewall height, between the top of the tread and the rim, is 55% of the tyre width. Many people get this confused and assume it relates to the tyre height between rim and tread, which it isn’t…
R16
The ‘R’ part here simply means that the tyre is of radial construction – don’t confuse it with the speed rating, which we’ll come to in a moment. The ’16’ part is the imperial bit – the wheel diameter to which the tyre will fit.
Some tyres will also carry a number and letter after this, such as ’91 V’, the 91 part being the load rating and the V being the speed rating.
The number relates to the weight each tyre can carry in kg, but is (of course) a source of further confusion, as it doesn’t mean the tyre can take only 91kg of weight. Load ratings start at ’75’, for 387kg, and rise to ‘150’ for plant applications, which can take 3,350kg. A tyre with a load rating of 91, which is a very common one, has a load rating of 615kg per corner.
The speed rating will generally be from N (87mph) to Y (186mph), although there’s also a ‘ZR’ rating for tyres rated above a ‘Y’. In the above example, a V-rated tyre is declared safe for speeds up to 149mph. You can see a full breakdown of these via tyre safety body, Tyresafe.
What about the other numbers?
Every tyre made after 2000 will carry what’s known as a DOT code – this is determined by the US Department of Transport, hence the name.
After the DOT code is the tyre serial number (which is simply its manufacturer product code, plus a letter determining in which factory it was built)
The most important one of these to check is the date code – a series of four numbers that appears after the serial number that refer to the week and year in which a tyre was made. So, for example, if the four-digit code is 3924, it means the tyre was made in the 39th week of 2024, or mid-October.
If a tyre only has a three-digit date code, then it was made before 2000 and shouldn’t be used on the road, as rubber perishes and that’s far too old. Before 2000, the decade wasn’t recorded in the date code, so a date code of 394 would mean the 39th week of 1984 or 1994 – perhaps only of relevance to historians or concours judges, but if you find this marking in your road car, do everyone a favour and change your tyres.
What about numbers on wheels?
Well, confusion reigns here yet again, for while we may measure the width of a tyre in mm, the width of a wheel is measured in inches. No, it doesn’t make sense – but it is what it is.
Most car wheels will be rated 5J-7.5J and the number represents the wheel’s width, so what’s the J for? That’s your ‘collar profile’, which is the shape of the flange where the tyre bead mounts, where the fitter puts the adhesive. It’s a pretty important element of any wheel, as not all tyres fit all collar profiles. The majority of cars are ‘J’, but some older models with taller profile tyres have a ‘P’ rating, such as a classic VW Beetle.
If you’re buying aftermarket alloys, the width and collar profile must be stamped into the wheel, along with the PCD.
PCD stands for Pitch Circle Diameter, which is a measurement of the diameter of an imaginary circle that passes through the centre of a wheel’s bolt or stud holes.
It’s given as a two-part number, such as 4×100, “meaning the wheel has four bolt holes and the diameter of the circle is 1oo millimetres. The measurement is critical for ensuring a wheel fits correctly on the hub.
That 4×100 is a very common PCD found on most small, European cars, one exception being Italian models, which commonly have either 4×98 or 5×98 as their PCD, which goes right back to when the Fiat 500 was built in its millions. The 98mm diameter was retained even on larger and more powerful cars, which were equipped with five wheel studs.
Other common sized PCDs on European cars are 5×100 and 5×114.3, found on many larger models.
When buying new wheels, you must match the PCD to your vehicle’s hub to ensure the wheel bolts will line up correctly – adaptors are available, but are frowned up on by many safety experts for adding an unnecessary extra union between hub and wheel.
Have you ever had a tyre or wheel sizing mishap? We’d love to hear your stories at hdc@hagerty.co.uk