Lesson 1: Locate the tyre info
This is easy. Head over to your tyre and stare in wonder at all the information on them. Now, locate the part that looks a bit like this:
Lesson 2: Tyre Width
The first 3 digits will tell you the width of the tyre. That is, the distance in millimetres from the inner sidewall to the outer sidewall. Note: the measurement is always done when the tyre is inflated and mounted on the rim size it’s designed for. So the width of a "245/45R17" tyre is 245mm.
Lesson 3: Tyre Profile
Next up is the profile (or ‘aspect ratio’ as it’s otherwise known). This is measured as the height of the sidewall as a percentage (%) of the width. The number featured is the %. So, in our example, the tyre's sidewall is 110mm high all around the rim. As the tyres width is 245mm, the profile is 45% (shown simply as "45").
Lesson 4: Tyre Rim
Now we get to the how big your rims are. This is normally expressed in inches, with the exception of some now very rare European fitments from the 90s. The numbers represent the measurement of the diameter of the rim. That is, the distance of a straight line passing through the centre of the tyre. And yep you worked it out correctly: a 245/45R17 fits on a 17" rim.
You'll usually find a letter ‘R’ between the ‘Profile’ and ‘Rim’ measurements. This ‘R’ points to the type of construction and stands for ‘radial’. Nearly all car tyres are a radial construction. If you're dealing with a very old trailer tyre, you might find the size is instead something like B78-13. The dash indicates that it's bias construction rather than radial. And the B78 indicates that it uses some other ancient form of sizing which we'll ignore for the purposes of this article. Back to the 245/45R17 which we have pictured.
Lesson 5: Tyre Load
The final number (although not the last piece of the ‘code’) is the Load Index. This tells you the maximum load the tyre can carry in KGs. To work it, you’ll need to consult a Load Index chart (like the one we've included here). Then you simply locate the numbers your tyre has and read across to find the maximum kgs they can handle. For example, tyres with a load index of 99 can handle up to 775kgs.
Lesson 6: Tyre Speed Rating
Well done. You’ve made it to the last bit of the code. Welcome to the speed rating. This tells you the maximum speed the tyres can handle. Of course, it would be too obvious to put the actual max speed on the tyre. Instead, you get a letter and need to consult another chart. A ‘V’ is rated to 240 km/hr. It’s worth noting... it doesn’t mean you can go this fast. You are limited to the speed limit of the roads you travel on. And you can only go as fast as the lowest rated tyre you have on the vehicle (your tyres may not all be the same). Check out a full chart of speed ratings here.
Test time: What does the following mean?
Yup, you got it. 205mm width, 65 profile and a 16 inch rim. Maximum load 365 kg, maximum speed 210 km/hr (or the maximum speed your roads are limited to, if you live in a country that has speed limits). It would be a pretty sweet tyre too as you can get a Continental UltraContact 6 in this size.
Hold the horses we here you say...this is all well and good but your car tyres have nothing like the numbers we’ve been talking about. Indeed, your numbers look more like 31x10.5R15. Fear not. It’s just the remnants of an imperial past, meant to confuse you. What they’ve done is not only express everything in inches but also mess with the order a little. If your tyre numbers look like this then they’re detailing the overall height (31 inches), width (10.5 inches) followed by ‘R’ (radial) and 15, which is the diameter of the wheel it's designed to fit.
That’s it. You can now congratulate yourself on a job well done and sit back in the comfort of knowing you ‘speak tyre’. You are now free to impress everyone from local tyre fitters, staff at Supercheap Auto, through to your mates down the pub.