Not sure what the finish is like on your wheel but I have used heavy duty toilet bowl cleaner in the past with very good results.May be overkill for your application though,some of them have about a 20% base of hydrochloric acid which eats through just about anything,maybe even the wheel itself if left on long enough.

Use gloves,wipe it on and be sure to rinse thoroughly...has worked well for me.Better to test a spot first,if you have dull gray areas it sounds like the chrome itself may have a problem.