Tire Load Index

The tyre load index is a numerical code moulded into a tyre's sidewall that indicates the maximum weight that single tyre is rated to carry when correctly inflated. It exists because a tyre's ability to support weight is not unlimited; it depends on the strength of its casing and belts and on the volume and pressure of the air inside, and exceeding that limit causes the casing to overheat, deform and ultimately fail. The index gives a precise, standardised way of expressing this capacity so that a tyre can be matched to the vehicle it must support.

The number is not the weight itself but a code that must be read from a standard reference table, in which each value corresponds to a specific figure in kilograms. The relationship is non-linear, so each step up the scale represents a slightly larger increase in capacity than the last; for example, an index of 91 corresponds to roughly 615 kg per tyre, 95 to about 690 kg and 100 to around 800 kg. The figure appears as part of the service description on the sidewall, sitting immediately before the speed-rating letter, so a marking of 91V combines a load index of 91 with a V speed symbol.

For the vehicle owner the load index is a genuine safety parameter rather than a technical curiosity. The combined capacity of all the fitted tyres must comfortably exceed the vehicle's fully laden weight, including passengers and cargo, so the manufacturer specifies a minimum index for each model. Fitting tyres with a lower index than required overloads the casing, generating excess heat that can lead to rapid wear or a sudden blowout, particularly when the car is heavily loaded, towing or travelling at speed in hot conditions.

The figures are standardised internationally by bodies such as ETRTO and their counterparts, so a given index means the same load capacity regardless of brand or country of sale, which allows tyres to be compared and substituted with confidence. It is worth noting that some commercial and light-truck tyres carry dual indices for single and twin-wheel fitment, and that reinforced or extra-load tyres provide a higher capacity at a given size for vehicles that need it.

A few practical caveats apply. The rated capacity assumes the manufacturer's recommended inflation pressure, so an underinflated tyre cannot safely carry its indexed load and is far more likely to fail. The index is always read together with the speed rating, the two forming the service description that defines the tyre's complete operating envelope, and it interacts with the choice between standard, run-flat and all-season constructions. When replacing tyres the sound rule is to meet or exceed the original load index, never fit below the vehicle's specification, and keep the rating consistent across an axle.