Tire rating on a tire goes from a-z Z being the highest. Same with ply rating A-E 2,4,6,8,10 ect
Z *used* to be the highest.... the industry made a mistake by trying top rate tires using letters though. Z = rated for speeds at or above 149MPH, Y = rated for speeds at or above 186MPH. See the link I posted earlier for more info, or just Google it.