Lifted from carbibles.com
Lies, Damn Lies and Speed ratings.
All tyres are rated with a speed letter. This indicates the maximum speed that the tyre can sustain for a ten minute endurance without coming to pieces and destroying itself, your car, the car next to you and anyone else within a suitable radius at the time.
Speed Symbol Max Car Speed Capability Speed Symbol Max Car Speed Capability
Km/h MPH Km/h MPH
L 120 75 S 180 113
M 130 81 T 190 118
N 140 87 U 200 125
P 150 95 H 210 130
Q 160 100 V 240 150
R 170 105 W 270 168
Z 240+ 150+
'H' rated tyres are becoming the most commonplace and widely used tyres, replacing 'S' and 'T' ratings. Percentage-wise, the current split is something like this: S/T=67%, H=23%, V=8%. Certain performance cars come with 'V' or 'Z' rated tyres as standard. This is good because it matches the performance capability of the car, but bad because you need to re-mortgage your house to buy a new set of tyres.