What do the size numbers on tyre sidewalls actually mean? A quick explanation
Let's take a common tyre size as an example: 195 65 R15 91 V:
195 - width of the tyre in millimetres.
65 - height of the tyre sidewall as a percentage of the width; in this case 65% of 195mm. This is also known as the aspect ratio. The lower the number, the more 'low-profile' the tyre.
R - this designates that the tyre is of radial construction, as nearly all new tyres are.
15 - the diameter of the tyre's inner rim in inches, i.e. the wheel size the tyre will fit onto.
91 - the load rating of the tyre. There's a table of rates and in this case 91 is 615kg
V - speed rating of the tyre. This indicates the maximum speed for the tyre when at full load. There's a standard table of rates, and V is 149 mph.