Why when you are driving down the road and a hub cap falls off does it go faster than the automobile it comes off?

Why when you are driving down the road and a hub cap falls off does it go faster than the automobile it comes off?
Conservation of Angular Momentum. When circular things rotate, they like to stay at the momentum they have at the current point in time. Any changes will affect either the mass or the rotation velocity (much more likely).
A hub cap has much less mass than a tire and rim combined. Prior to falling off the rim, it was rotating at the same speed. When it fell off, it wanted to keep the same momentum that it had when attached to the whole wheel. But since it has less mass, it need more VELOCITY to compensate. (momentum=velocity X mass)

Because of this simple physical law, the strange effect is achieved and the hub cap will actually pass the car instead of slow down. It will slow down eventually because of air resistance but this is why it appears to accelerate first.