If one watches a few more episodes of the show, it would become obvious that the Prius fuel consumption test was a joke, like everything else in the show. Come on, we can do better than expecting a fair review of a hybrid car from a guy who comes up with increasingly witty descriptions of his erections caused by the power and speed of various sports cars.
Edit: it probably didn't come across like it, but I think that the show is hilarious and it is one of the very few reasons to own a TV. But a car review show (as in, the source of information on the automotive industry) it is not, and is not meant to be.
Compared to a typical European car, such as the Diesel Passat station wagon, the Prius IS a joke. It has worse fuel consumption, carries less, and has a massively more complex drive-train.
Sure, the Passat is a bit of a slug, but light-footed, I made an estimated 50 mpg in Italy, 4 up with luggage, AC on, during an Italy trip, something you couldn't do in a Prius.
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.
Exactly the point I took away from that test, it matters much more how you drive (calmly rather than aggressiely) than the car that you drive. This was even confirmed on mythbusters in their driving calm versus driving angry 'myth'.
While I understand that this segment of theirs was a joke, it did have a point to it. They were trying to show that it is possible to drive a Prius in such a way that you don't actually save any fuel over a non-hybrid.
I don't think anyone would dispute that a Prius is more fuel efficient than a sports car in stop-and-go traffic and having that segment on Top Gear just wouldn't be possible as it would make the show "fair" to the hybrid and actually about cars which it definitely isn't.
And I think they make these silly claims at times since they know they are so over-the-top that it's hard for anyone to take them too seriously.
Their test was to loop non-stop around a race track at the Prius' top speed, which is pretty much what the sports car was designed to do.
They neglected to do a round in stop-and-go traffic, which is what the Prius was designed for.