The Load Index is a numerical code associated with the maximum load a tyre can carry at the speed indicated by its Speed Category symbol under specified conditions up to 210km/h.
Using a 265/75R16 119N tyre size as an example, the 119N at the end of the size represents the tyre's service description. A service description identifies the tyre's load index and speed rating. The first three digits - 119 - represent the tyre's load index and are followed by a single letter - N - identifying the tyre's speed rating.
265/75R16 119N - The Load Index - 119 - is the tyre's assigned numerical value used to compare relative load carrying capabilities. In the case of this example the 119 identifies the tyre's ability to carry a maximum of 1360 kg (2998 lbs). The higher the tyre's Load Index number, the greater its load carrying capacity. The Load Index rating also represents the load carrying capacity of the tyres when they are inflated to maximum psi so the load carrying capacity reduces as tyre pressures are reduced.
Speed ratings are based on laboratory tests where the tyre is pressed against a large diameter metal drum to reflect its appropriate load, and run at ever increasing speeds until the tyre's required speed has been consistently exceeded.
New Zealand Vehicle Standards' rules state that a 4WD manufactured with off-road features must have a tyre speed rating equal to, or greater than:
(a) the maximum legal speed for the motor vehicle to which the tyre is fitted; or
(b) the vehicle's maximum speed.
It is important to note that speed ratings only apply to tyres that have not been damaged, altered, under-inflated or overloaded. A tyre that has been cut or punctured no longer retains the original speed rating, even after being repaired.