I have a multifuel stove in my living room and the Stovax product I use for the glass plate is excellent for softening and removing both the caked on crap on wheels from the brake dust and also tar, soot, etc., that accumulates around the tailpipe rims ( I have a diesel).
Stovax do a gel formulation which you can leave on and come back to 10mins later and its still there. It's 6 quid a bottle and lasts me a long time.
And as AshTDiFR says, it justs jetwashes off along with the grime
I lent it to my neighbour today to do his BMW 735. His front wheels were almost black even after a jet wash. With Stovax gel and a toothbrush, they came up as good as new - no kidding!