Circular Preloader

I have an animation that is contained within a movieclip. It is an animation of a car travelling round a globe. The car is set to a circular motion guide. I would like to use this as a preloader. What is the best way to get an animation to play in time proportionally as it is loading. i.e the animation finishes when the site loads. If this is not possible what other methods are there to create circular loaders as opposed to linear bars that increase in size.

Thanks