How do I read the markings on the sidewall of my tyre?

Every tyre shows information pertaining to its manufacturer, size, model etc. The following is an explanation of all the terminology used for both regular and low profile tyres;

Typical example: 165 R 13 79 T

165 - means the tyre has a nominal section width of 165 millimetres
R - means radial
13 - means it fits a 13" diameter wheel
79 - Load index
T - means it's rated for speeds up to 118mph (l90kmh)

Other common speed ratings are:

S = 113mph
T = 118mph
H = 130mph
V = 149mph
Z = Over 150mph
W = 168mph
Y = 186mph

You should always replace a tyre with the same or a higher speed rating. In general, the higher the speed rating, the better the quality of the tyres and the more advanced the technology. This is especially true in areas such as steering response, wet grip, cornering power and braking adhesion.

Load Index

Some vehicles require tyres that are rated to carry a higher load and therefore have a higher inflation pressure. This information is contained in the manufacturers handbook and the tyres will carry the marking RF (Reinforced) or XL (Extra Load)

Low profile tyres have a slightly different marking for instance:

Typical example: 185/60 R 14 79 H

185 - 185 millimetres nominal section width
60 - sidewall height is 60% of tyre nominal section width
R - Radial
14 - Means it fits 14" diameter wheel
79 = Load index
H - Rated to 130mph

A standard tyre has a sidewall height, which is 82% of tyre nominal section width.

Reducing the % of sidewall height to tread nominal section width (the 'aspect ratio') enables tyres to be wider and put more tread on the road.

Can I increase the size of my tyres?

It is possible to increase the size of your tyres (subject to the manufacturers recommendations).

In general, for every 10mm increase in tyre width i.e. 165 to 175, a reduction of 5mm in the sidewall height is required i.e. 70 to 65

For example, a 165/70 R13 T would become a 175/65 R13T