Are you talking normal WW's or WWW's ? Cuz on my WWW's I use Westleys and a curved tire brush.. I think it would take me forever if i used one of those magic erasers..
I spray it..scrub it..rinse off.. Can do both tires in 5 mins or less.
Cleaned mine so much the white came off.
Seriously, I used to use Westley's.
Magic eraser is more abrasive than you might think. Try it on a piece of shiny white fiberglass (boat).
I use the Whesly's on my WWW but I don't think the actual white wall on this tire is made out of rubber. It kind of feels like vinyl and the Bleach Wite stuff doesn't react on it the way it does on the black part of the tire. I still use it but it's not perfect. I use a scrub sponge with the green scotchbrite on one side. I have scrub the devil out of it. The Bleach Wite does make the black part of the tire blacker, which has the optical illusion of making the white part look whiter.