For example, if the tyre has a speed rating of V, then the maximum speed it can sustain is 149 mph (240 km/h).
Can I switch to a tyre with a higher speed index?
When your tyres are worn out, it is perfectly possible to replace them with tyres with a higher speed index than the one indicated on the old ones. However, you should never mount tyres with a lower speed index unless fitting tyres with winter capacities such as winter tyres or All Season Tires, as discussed below.
Choosing a lower speed index for winter tyres
If you use tyres with winter capacities (marked 3PMSF : either winter tyres or All Season Tyres), the speed rating may be lower than that of your summer tyres or the original tyre fitted to your vehicle.
Why is this possible? Tyres with winter capacities are designed differently for driving in difficult climatic conditions (snowy, icy roads, etc.) and may not have the same speed rating.
However, generally you drive at a lower speed in winter than you would on dry roads with summer tyres. In the UK the speed rating of tyres with winter capacities will still be in excess of the national speed limit. In any case, the driver must limit the speed of the car so not to exceed the maximum speed of the tyre.