Are you confused by your car's tyres? Don't know your rolling radius from your radial? Then take a good long look through this page where I hope to be able to shift some of the mystery from it all for you. At the very least, you'll be able to sound like you know what you're talking about the next time you go to get some new tyres.

Tyre sizes and what they mean.

Okay, so you look at your car and discover that it is shod with a nice, but worn set of 185-65HR13's. Any tyre mechanic will tell you that he can replace them, and he will. You'll cough up and drive away safe in the knowledge that he's just put some more rubber on each corner of the car that has the same shamanic symbols on it as those he took off. So what does it all mean?

185	65	H	R	13
This is the width in mm of the tyre from sidewall to sidewall when it's unstressed and you're looking at it head on (or top-down).	This is the height of the tyre sidewall, or section height, expressed as a percentage of the width. It is known as the aspect ratio. In this case, 65% of 185mm is 120.25mm.	This is the speed rating of the tyre.	This tells you that the tyre is a radial construction. Check out tyre construction if you want to know what that means.	This is the diameter in inches of the rim of the wheel that the tyre has been designed to fit on. Don't ask me why tyre sizes mix imperial and metric measurements. They just do. Okay?

More recently, there has been a move (especially in Europe) to adjust tyre designations to conform to DIN (Deutsche Industrie Normal). This means a slight change in the way the information is presented to the following:

185	65	R	13	91	V
Tyre width	Sidewall height %	Radial	Rim diameter	load rating	speed rating.

Decoding all that information on the sidewall

It's confusing isn't it? All numbers, letters, symbols, mysterious codes. Actually, most of that information is surplus to what you need to know. So here's the important stuff:

	Key	Description
	A	Manufacturers or brand name, and commercial name or identity.
	B and J	Tyre size, construction and speed rating designations. Tubeless designates a tyre which requires no inner tube. See tyre sizes and speed ratings below.
	C	Denotes type of tyre construction.
	D	M&S denotes a tyre designed for mud and snow. Reinforced marking only where applicable.
	E	Load and pressure marking requirement (not applicable in the UK). These go from a load index of 60 (250kg) up to an index of 114 (1180kg).
	F	ECE (not EEC) type approval mark and number.
	G	North American Dept of Transport compliance symbols and identification numbers.
	H	Country of manufacture.

Calculate the speed difference

Current wheel/tyre	New wheel/tyre
/ R	/ R


Current RR:mm	New RR:mm
Current circumference:mm	New circumference:mm



Difference in circumference:mm or %
So when your speedo reads 70mph, you're actually travelling at mph