Z is the highest - usually reserved for serious sports cars... Y is just behind it with a max speed rating (1 hour use) of 300km/h. W @ 270km/h and V @ 240km/h.
I got the Potenza V's and there great.
I would think from R to Y rate yes, but not at the end of the alphabetical
scale. The difference between V and W was R500 per tyre...