When I mobile detail, and since it's winter, I bring along a 5 gallon jug of water with spout. Then I set it on a table (at the client's site) and fill water spray bottles as needed, buckets as needed, etc. It's very efficient. For tires, I spray Westley's Bleche White dilluted about 1 parts Bleche White to 3 parts water and cover the wheel as best as I can with a rag while spraying. Then I use my spray bottle (of water) to thoroughly rinse (as best as I can) the cleaner off and wipe with a rag. I know Westley's can be dangerous but I dillute and where rubber gloves. The results are pretty awesome, although I've never used another tire cleaner (which I'll be buying with my next order online).
For wheel, I use Sonus Rim Bright and Spray the wheels, let sit, then thorough rinse (as best as I can) and wipe away solution, repeating until the water I wipe rinses clean without any Wheel Cleaner in it. If the wheel has grooves (by lug nuts), I aim the water bottle right up and spray. This would probably where you'd want a pressure washer...but it works for me for now until I break even and have money to buy more goodies:xyxthumbs