Load index (LI) is one of the most important pieces of information on the sidewall (next to the size and the speed index). Load index determines the load allowed at the maximum speed by the tyre manufacturer for a specific tyre. Both values are given on every tyre.
You can find the recommended load index for your vehicle in the service book.
There are many ways to match the load index to your car. If the tyres have a load index of 90, that means that the load per tyre should not exceed 600 kg. If you multiply this value by the number of tyres on the car, you should get a value which is slightly higher than your vehicle's maximum allowed weight (with full load). This information can be found in your registration certificate (field F1).
Fitting tyres with a lower load index than is recommended by the car manufacturer is not permitted: it could have serious consequences and it is illegal. The tyre will wear faster and cause instability, and the tyre may even explode while driving, which could cause a very serious accident.
The table above shows the weight permitted for each LI rating.