If you are looking to understand what tyre numbers mean, in this post we break it down nice and clearly. There’s quite a few reasons why you want to understand what the tyre numbers mean, and if you are in the market for a new set of tyres it greatly helps to have a basic understanding, so you don’t get ripped off and you get the tyres that are going to suit your requirements.
We start off with tyre sizes, and then move to load ratings, speed ratings, date of manufacture and lastly tyre pressures in a simple, easy to understand way.
Metric vs imperial tyre sizes
Car and 4WD tyres are sold in sizes that will either be in metric or imperial, and it can be quite confusing to work out what they actually mean. In both cases you get 3 sets of numbers, but they are vastly different from each other.
Metric tyre numbers
A standard tyre on a 4WD today is a 265/75/16. What on earth does that mean, I hear you ask? It’s actually far less complicated than you might think.
The first number (265) is the sectional width. This is literally the width of a tyre, so a 265 tyre is 265mm wide, from the sidewall to sidewall at its maximum point.
The second number is the aspect ratio, which is given in a percentage and refers to the height of the tyres sidewalls. In this case, the aspect ratio is 75% of 265mm, which means 265*0.75, giving you 198.75mm.
The last number is the wheel size, which in this case is a 16 inch wheel, or rim.
In essence then, you have a tyre that is 265mm wide, 198.75mm tall at the sidewall and suits a 16 inch rim.
Imperial tyre sizes
Imperial sizes are even easier to interpret, and give you a easier to picture result, but its all in inches, and not mm or cm.
If we talk about a 33/12.5/16, you get a tyre size not too dissimilar to the one above (its slightly larger).
The first number is the tyre diameter, being 33 inches tall. This is the total height from the bottom of the tyre to the top of the tyre.
The second number is the width of the tyre, which is 12.5 inches wide, from one sidewall to the other in its maximum point.
The last number is wheel size, and in this case suits a 16 inch wheel or rim.
Converting from metric to imperial (or imperial to metric)
If you are wondering what a 265/75/16 is in an imperial tyre, or what a 33/12.5/16 is in a metric tyre, you can manually work it out, or the easiest way is to just use a tyre size calculator.
To manually work it out, you have to convert it from one to the other. If you are going from metric to imperial, divide the size by 25.4. If you are going from imperial to metric, multiply it by 25.4.
For example a 265/75/16 is
Width is 265/25.4 (10.433 inches wide)
Tyre height is 75% of 265, which is 198.75*2/25,4 plus 16 inches giving you a total result of 31.65 inches tall
Suiting a 16 inch wheel
A 33 x 12.5 x 16 is
33 x 25.4 = 838mm tall
12.5 x 25.4 = 317.5mm wide
Suiting a 16 inch wheel
Common tyre sizes
We always recommend that you try and run tyres that are a common size. You can get some weird and wonderful wheel and tyre combinations today, but the less common they are the more you’ll pay, and the harder they are to get. I recently heard of a couple of people trying to get tyres to suit 20 inch wheels in the Kimberley, and they were having zero luck at all.
Your tyre shop will be able to tell you what the most common tyre sizes are, and you’d be wise to try and run them where possible.
Fitting bigger tyres
A lot of people swap their tyres out for larger ones, and that’s fine, providing you understand what happens when you do so. We have a comprehensive article that covers it in detail; fitting bigger tyres to your 4WD. This goes into the changes that take place to your torque, power, fuel consumption, insurance, centre of gravity, clearance and heaps more.
Load rating on tyres
Every tyre will have a load rating stamped on it, which gives you the load index and can be translated into the kilograms allowed on each tyre. For example, our camper trailer tyres are a 123 load index, which means they are allowed 1550kg of weight applied on each tyre.
Speed rating on tyres
On top of the load rating, you’ll have a speed rating, which is specified by the vehicle manufacturer, and is the minimum that you can comply with. They start off at N, and work their way to Y, which ranges between 140 and 300km/h.
- N – 140
- P – 150
- Q – 160
- R – 170
- S – 180
- T – 190
- U – 200
- H – 210
- V – 240
- Z – Over 240
- W – 270
- Y – 300
Date of manufacture
Inside a little box on the sidewall will be 4 numbers, on every tyre. This is the manufacture date, and is actually really important to check, as tyres deteriorate both with use, and age. The first two numbers are the week of the year, and the second two numbers are the year.
For example, 4021 would mean that they were made in the 40th week of 2021. If you see tyres with an age stamp that is more than 5 – 7 years old, you should be cautious of it. You can read more about this here; tyre age.
Lastly, there’s figures given on the sidewall of each tyre that show the maximum inflation pressure when the tyre is cold. That means the tyre pressure when the tyre hasn’t been used for several hours, preferably after sitting over night.