Westley's Bleache-White. Soft scrub brush for the whole sidewall, then a wire whitewall brush on thw whitewall or letters. I scrub blackwalls the same way. Then dry 'em and put Armor All on.
I go through a lot of it, so I buy a gallon of Westley's Concentrate when I can find it. Costs about $9.00, you mix it 50/50 with water. A bunch cheaper that way. It also cleans the gray cladding up nicely, then top that with Mother's Back To Black. My daily-driven year-round vehicles have better looking tires than a lot of show cars I've seen.