If the wheels are dirty, so are the tires, you just can't see it. Give the tires a good scrubbing too until you don't see brown foam. The silicone spray you mentioned Paul is one of my best detailing tricks. Look for it with the foamy tire cleaners. Its a black can with silver writing.
Silicone puts a perfect sheen and re-moisturizes the rubber.
Once every 3 months, take the wheels off the car and clean the inside of each wheel. Best of all, many of us have that whited-out look on the rubber strips around the mirrors. I almost replaced my mirrors until this trick. Next day, spray lightly, and wipe, rubdown the rubber to a new gloss. Same trick works on Bumpers too.