Is it a German car? =P
The reason I ask is that I see that ALL the time on BMWs with those little black specs all over the rim from going more than a week without washing them. Really, the only way to get rid of it is to clay, clay, and just when your arm is about dead, clay just a little bit more and bam! You have finished your first wheel!
I am not joking either, expect to spend anywhere from 20 minutes to 1 hour per wheel in just claying to get rid of that stuff. Now, this is on Mercedes/BMW/Audi cars with factory brakes, so maybe yours won't be as bad, but I am just preparing you for the worst!
Just FYI, I have tried heavy acid cleaners, and about the only two wheel cleaners that work are Meguiar's Wheel Brightener (for factory coated wheels) or P21s if the wheels are bare aluminum or anodized. Even with these, it is often required that you clay the wheels for the real bad stuff. Hope that helps!