Actually, had you bothered to watch to the end of that episode, the point they made was about HOW a car is driven and now WHAT car you drive.

They drove the Prius as fast as it would go, because someone who buys a Prius will do that and at that speed it would be less efficient than an M3 which is the "sports car" you mention.

And I agree with that point. If I flog my Civic, it'll return much smaller mileage than if I drive carefully and efficiently. So, again, Top Gear's point is proven in that, people aren't willing to think critically and live in a reality.