Tyre Designation

Ply Rating "PR" (bias tyre)

Used to be the number of cotton ply layers in the carcass. With new cord materials (nylon, polyester, steel etc) the actual number of layers is less than stated.

Star marking (conventional radial tyres) - used mostly on USA made tyres

1 Star: rated at 18psi/1.2 bar.

2 Star: rated at 24psi/1.7 bar.

3 Star: rated at 30psi/2.1 bar.

Load index "LI" and Speed Index "SI" (metric tyres)

States the maximum load at given speed and inflation pressure.

"LI" code stands for a given load (eg. LI 128 = 1800 kg).

"SI" code stands for a given speed (eg. A8 = 40 kph, D = 65kph).

Outer diameter: The diameter of an inflated tyre at the outermost surface of the trend.

Section width: The linear distance between the outsides of the side walls of an inflated tyre excluding elevations due to labelling (markings), decorations or protective bands or ribs.

Section height: Half the difference between the overall diameter and the nominal rim diameter.

Static loaded radius: Distance between the rim centre and paved surface.

Rim width: The linear distance between the flanges of the rim.