1. Be SO happy they are not chrome or painted steel.
Then there are 2 routes. use a jet wash. (not really recommended) or...
2. Accept the best way is to remove the wheels, so you will need a nice low level jack and axle stands.
3. Go to your garden centre and buy a plastic patio pot base that is large enough to sit under the tyre.
4. Go to Halfords and get stuff for cleaning motorbikes. Muckoff or similar.
5. Take off wheel, put the "tray" on something like a workmate at full height and put the wheel on it.
Then it is easy, once you have done it the first time (took me about an hour a wheel because they were filthy, with excess grease running up the spokes, etc, etc.)
Now it takes me about 15min per wheel and you know both sides are clean and bright. You also get to make sure the splines are properly greased, there are no lose or broken spokes and all is in good order.
