Don't be soo quick to assume it is tar. It can be, but if results don't shine after the tar remover, use an acid based wheelcleaner. What I think it might be, is actually brakedust, especially if you mention its primarily on the front wheels. They obviously get the most brake force. When it accumilates the brake dust gets bad, and along with heat they sort of bake into the wheel. Brake dust is small metal aprticles. Wheel cleaners with acid ingredients melt away the metal to a point that it becomes a lot easier. Give it a try.
A good strong OTC acid-based cleaner is
Meguiars Hot Rims.
Joy.