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Wesley's Bleach White is the only way to go. Spray it liberally on a dry tire surface. Scrub it with a hand brush and let it sit for about a minute. Go over it with the brush again and rinse it off with water. Perfect! Where lots of folks create problems is that they use Armor-all or some other petroleum based product on their tires. It eats at the rubber and discolors the white letters [or whitewalls]. I use Nu-Vinyl on my tires. It dries clear and isn't oily and the tires look like new rubber (not glossy).
Never, never use Armour all for tires, really yellows fast! I have wide whites on my Harley, use two things only, sos pad lemon fresh if real dirty and comet with a straw brissel brush for general clean. Niether will spot aluminum or chrome rims. Same goes for white letter tires,been doing it for years....
I use Spray 'n' Wash laundry stain remover (stain stick). Put it on the letters, scub a small effort, let stand while you wash the rest of the car and rinse off. The letters really look great with little effort.