Tire ratings...
N=87 MPH, 140km/h, Spare Tires
P=93 MPH, 150km/h ans
Q=99 MPH, 160km/h, Winter, LT Tires
R=106 MPH, 170km/h, LT Tires
S=112 MPH, 180km/h cars
T=118 MPH, 190km/h
H=130 MPH, 210km/h, Sport Sedans
U=124 MPH, 200km/h
V=149 MPH, 240km/h, Sports Cars
W=168 MPH, 270km/h, Exotic Sport Cars
Y=186 MPH, 300km/h, Exotic Sport Cars
Z=149 MPH, 240km/h and over, Sports Cars
I guess you need to determine how fast you are going to go and how long you are going to sustain that speed. Then chose the tire rating to fit your needs.
These ratings are for normal passenger cars.
Mike
Info from:
http://www.tirerack.com/tires/tirete...eral/speed.htm