in most games that I've seen tanks in, they use an animated texture to simulate the treads moving... so, the tread itself is actualy just a big oval piece. that doesn't really move on its own volition.

Once you get the main body of the tank done, you'd have a good idea how many polys are left to afford to the treads. Don't be afraid to sacrifice detail... that's the name of the game when it comes to game modeling. Trying to get a good ratio of geometry : detail can be tough.