The dirt on your wheels and tires is a chemical mix of iron from the brake drums or discs, dust from the calipers and a combination of other chemical residue from roadsplash.
Though it may look like ordinary dust, wheel and tire grime is certainly not. To make matters even worse, this chemical cocktail is baked on by the heat from braking, friction heat from the tires and sun exposure. This makes this stubborn dirt even harder to remove. For fast, thorough results, use a wheel and tire cleaner. They are specially formulated to efficiently tackle this job.