Welcome to the second part of my explanation about 4wd rims and tyres. In my previous post explaining 4wd Rims, I mentioned that I run 285/75/R16s on a 16 x 7.5 steel rim with a +10 offset. I don’t by the way, but it’s a perfectly valid example for us to use…

If you’ve read the previous post then you’ll understand the last part… 16 x 7.5 steel rim with a +10 offset. In this post, I’ll explain the rest of it and also some other common terms that you’ll hear regarding tyres.

# What does the rest mean?

Tyres are measured in a couple of different ways; imperial (inches) and metric (millimetres). The example above is the metric system… Kind of… Except that some of it is in inches… Crazy I know!!!

So what does it mean? Ok. Lets get into it. Its actually not that difficult to understand.

Tyres are measured in width of the tread, side wall size and rim size. In our example:

- 285 is the width of the tyre in millimetres.
- 75 is the height of the sidewall as a percentage of the width of the tyre. Otherwise known as the Aspect Ratio.
- R16 is the size of the rim it’ll fit in in inches.

So there you go. Simple. Thanks for coming. Wait… what? You want more? Ok then…

# Calculating tyre diameter

So lets say you want to know the rolling diameter of our tyre. There are two ways you can do this. Lets have a look at the first method.

In our example, our tyre is 285mm wide. The sidewall is simply 75% of 285 (285 x 0.75) which is 213.75mm high. But that still doesn’t give us the full diameter, it only gives us the height of the tyre. The distance from the rim to the road, so to speak.

To get the full tyre diameter, we need to also add the rim size. In our case it’s 16 inches. There are any number of places on the internet to convert inches to millimetres. But I’ll do it for you this time. Just this once, ok.

16 inches = 406.4mm

So our total tyre diameter is 213.75mm + 406.4mm + 213.75mm = 833.9mm

When talking about tyre diameter, most people talk in inches for some weird reason. So then we convert 833.9mm back into inches… Which comes to 32.83 inches.

The second method would be to visit a website that does it all for you. Like this one at RimsNTires.com, or this one at ExploreOz.

But hang on… You’ve seen tyres measurements that don’t seem to look like this at all! You’re right! Sometimes tyres are measured only in inches!

So for example your tyre might be a 33×11.5R16. In this case, 33 inches is the total diameter of the tyre in inches. 11.5 is the width of the tyre, and R16 is the rim size. If you did the calculations, you’d find that it’s very close to our original example of 285/75R16

# Other tyre talk

Some other terms you’ll hear when people are talking about tyres are:

**Carcass** – According to etyres.co.uk, the tyre carcass is basically the black bit, including the tread, the sidewall, the steel belts if there are any and everything else.

**Tread** – This is the bit that keeps you stuck to the road. There are all different kinds of tread for all different types of terrain.

**Sidewall** – This is the side of the tyre. The bit you kick when you’re telling people about your tyres. it keeps the tread away from the rim.

**Bead** – This is the part of the tyre that comes into contact with the rim. You’ll hear people talking about popping the bead off the rim. This basically means that their tyre has come off the rim and all the air has escaped. It can be difficult to get it back on again if you don’t know what you’re doing.

**Footprint** – This is the area of the tyre that comes into contact with the ground. One of the best 4wd tricks to get you further is to reduce your tyre pressure. This increases the foot print of your tyre and gives you more traction.

**Profile** – The tyre profile is the aspect ratio. The height of the sidewall.

That’s basically it really. There are many more terms that you might come across but these are probably the most common.

*Feature Image from ExploreOz

In the 33×11.5R16 example above Your text surgesting that the 11.5 refers to the tyre height would meen that the rim is a 10″ rim. I think that the 11.5 refers to the width of the tyre

Y’know… According to this website: http://indonesia4x4.blogspot.com.au/2006/08/tyre-sizes-explained.html

You’re right. I’ve updated the article now. Thanks for pointing that out.

Cheers

RossH

In your example above you have “The sidewall is simply 75% of 285 (215 x 0.75) which is 213.75mm high”. I think you mean (285 x 0.75).

Also could you explain the size 7.50 x R16 110/114

Hi Bob

The 2nd set of numbers is the load/speed rating. You can read more about it here: http://lindsaystyres.com.au/load-speed-ratings/

And from what I can work out, a tyre that is sized like yours (7.50XR16) means that it has a 100% profile. So the height of the tyre sidewall is the same as the width, in your case I’m guessing 7.5 inches.

Does that sound right?

Cheers

RossH