I use Comet for the white walls on my Harley (Don't have WW's on my MB). Wet the tire, shake the Comet powder on there, scrub with a coarse brush till it has a paste on there, then let sit and scrub again, rinse. May take 2 tries with REALLY Dirty white walls (Like after windy mountain runs xP) But it get's em clean! Wife asks, say you got it so SHE can clean the tub! xP
I've also used Trisodium Phosphate (TSP). It's an industrial cleaner I use to prep for painting and for my sneakers! Mix a little with water, maybe 1/4 to 1 ratio and scrub. Rinse hands after use thou, after about 5 minutes it will iritate and possibly burn. but gosh does it clean!