What does load index 100 mean?
Answer
1,764 lb per tire
Load index 100 means one tire is rated to carry 1,764 lb — 800 kg — at the inflation pressure printed on its own sidewall. Four of them total 7,055 lb.
The working
- Per tire
- 1,764 lb / 800 kg
- Four tires
- 7,055 lb
- One index step up
- 1,819 lb
- One index step down
- 1,709 lb
800 kg, and where that sits on the chart
Index 100 is one of the round numbers in a series that is otherwise full of awkward ones. It marks roughly the point where passenger-car indexes end and crossover and light-truck indexes begin: a great many family cars carry indexes in the eighties and nineties, and a great many SUVs start at or just above 100.
Four of them add up to more than the vehicle weighs
7,055 lb across a set, which comfortably exceeds the gross weight of anything likely to be wearing them. That headroom is not waste. The index is measured at a specified inflation pressure under standardised static conditions, and a vehicle in motion does not distribute its weight in four equal quarters — a corner under braking or cornering load carries considerably more than its share. The number that reflects what the vehicle was designed around is on the door-jamb placard, not on the tire.
The step either side
Index 99 is 775 kg and index 101 is 825 kg, so one point is 25 kg here. That is a little over 3% per step — finer resolution than the series uses higher up, where the increments widen to 35 and then 50 kg. Ratings are not proportional to their index number in any simple way, and the gap between two indexes has to be read off the table rather than inferred.
An index is a property of the tire, not of the size
Two tires in the same size code can carry different load indexes, because capacity comes from construction and reference inflation rather than from the dimensions. An Extra Load version of a passenger size is rated at a higher pressure and consequently carries a higher index than the standard version of the identical size. Looking up an index from a size code alone is therefore not possible, and any chart that claims to do it is generalising.
Reading it in the wild
An index of 100 appears in a service description such as 100T or 100H, immediately before the speed symbol. On a light-truck size it can also be the single-wheel half of a paired marking. Feed a complete description to the decoder for both halves at once, and the load index table has the full series if you need a neighbour.