Friction. Both through air resistance and through the top's point of contact with the surface below. In space, with no resistance, the top would spin forever, and that would be true whether it was light as a feather or as heavy as an ocean liner.Hey redpoint5, unless I have misunderstood your point, you make an interesting case for perpetual motion. I would rather not start discussing a comparison between the motion of a rocket travelling to the moon and the motion of a car travelling on earth, as I don't believe they are comparable. But if I take your interpretation of the physical world, then I might ask why a spinning top will eventually come to rest?
The same is true of the wheels on a car. Once they're up to speed, their mass is irrelevant in terms of maintaining their angular momentum - the only thing that matters is bearing friction, air resistance, rolling resistance, and other frictional losses.
Now I suppose you can argue that on a rough surface where the suspension is being given a workout, the lower mass of the wheels might translate to a lower rolling resistance because the wheels would conform more easily to the undulating road surface. But even if that was true it would be pretty negligible compared to the other losses.