I use oven cleaner spray containing Potassium Hydroxide on very soiled wheels.
I buy it from a professional household cleaning supply shop in plastic spray bottles.
Spray on, leave for 2 minutes using brush to loosen dirt, jetwash rinse off.
It does a fantastic job on grease, brake residue, tar.
I have used it on aluminium, painted aluminium and steel wheels and if you take care not to leave it on for more than 6 minutes no damage to the wheel is done.
It even degreases the engine compartment. Dont leave it on plastic or rubber surfaces too long, just in case, not that I have seen any damage in the last 2 years I've been using it.
I recommend trying it on the inside of the wheels first.
It turned my yellowing Webers into bright silver aluminium and took all the brown petrol gunk off.
Use with care, it is a nasty chemical, ALWAYS use rubber gloves and eye protection especially when jet cleaning after application.
Use in an open area and conform with your local EP laws on chemical disposal.