Z used to be the highest, now cars can exceed that so there are a few more speed ratings.
N=87 MPH, 140km/h, Spare Tires :D
U=124 MPH, 200km/h
P=93 MPH, 150km/h
H=130 MPH, 210km/h, Sport Sedans
Q=99 MPH, 160km/h, Winter, LT Tires
V=149 MPH, 240km/h, Sports Cars
R=106 MPH...