Right, so you're tyres are worn and you need to replace them.
A quick check in your manual, on the internet, or you bend down and read the sidewall of the tyre ... and you find you need a set of 185/65 R13 T'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 you've got new rubber on each corner of the car that has the same runes and  symbols on it as those he took off.

But what does it all mean?

 

A Renault Clio 1.2 has 175/65 R14 T tyres.

 

175 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 known as the section width.

 

The next figure 65 in the case of the Clio, is the Aspect Ratio. This is the height of the tyre sidewall, (section height), expressed as a percentage of the width.  In this case the section height is 113.75mm which is 65% of 175mm.

 

The R tells you the tyre is a Radial.

14 is the wheel rim size in inches.  (Why do tyre sizes mix imperial and metric measurements? Because that's the way it's done)
If you need hub caps you'll need to know this measurement.

Finally on our Clio we have the letter T ... This last letter is the speed rating of the tyre. 

 

All tyres are rated with a speed letter. This indicates the maximum speed that the tyre can sustain for a ten minute endurance without coming to pieces and destroying itself, your car, the car next to you and anyone else within a suitable radius at the time.

 

Speed Symbol	Max Speed Capability		Speed Symbol	Max Speed Capability
	Km/h	MPH		Km/h	MPH
L	120	75	S	180	113
M	130	81	T	190	118
N	140	87	U	200	125
P	150	95	H	210	130
Q	160	100	V	240	150
R	170	105	W	270	168
			Y	300	186
			Z	240+	150+

 

'H' rated tyres are becoming the most commonplace and widely used tyres, replacing 'S' and 'T' ratings. Percentage-wise, the current split is something like this: S/T=67%, H=23%, V=8%. Certain performance cars come with 'V' or 'Z' rated tyres as standard. This is good because it matches the performance capability of the car (obviously impossible to test in Gibraltar), but bad because you need to re-mortgage your home to buy a new set of tyres.