Dawn dish soap is one of the best degreasers (and cheapest) out there. Squirt some directly on the tire, scrub a bit with wet brush or wet sponge to get a lather going... then hose off until everything is removed, clean, and clear.
I use the same thing to clean grease off other things... mower, grill, bike, etc.... It works great!
You can dilute it for "less greasy" jobs... a teaspoon or two in a half-gallon of water... and scrub away!
2002 VT1100C2 Sabre
1985 VT700C Shadow
1999 CB750 NightHawk (son's ride)
"The Hardest Part About a Zombie Apocalypse, Will Be Pretending I'm Not Excited