What does speed class mean?

Your car's registration document states which speed class of tyres is suitable for the car. Car manufacturers determine the tyre size and speed class so that they correspond to the car's structural top speed and to ensure that the tyres are compatible with the chassis. The speed class is indicated by the second-to-last letter in the tyre marking. For example, the marking 205/55 R 16 94 V XL has a speed class of V, which indicates that the tyre has a maximum permitted speed of 240 km/h. If the car's structural speed is 240 km/h, EU regulations state that the car cannot be fitted with tyres of speed class H, which indicates a maximum speed of 210 km/h.

The most common speed classes are:
Q = 160 km/h
R = 170 km/h
S = 180 km/h
T = 190 km/h
H = 210 km/h
V = 240 km/h
W = 270 km/h
Y = 300 km/h