Fuel mileage comparison by Top Gear

The Toyota Prius gets 17.2
the M3 BMW gets 19.4

When tested at the Prius top speed around a race track, and the BMW following at the top speed the Prius could do around the racetrack.
No joke, Top Gear, Season 11 episode 1, 8 minutes into the episode.

At racing speed around a track, worst to best mileage of supercars as determined when they ran out of the one gallon of gas they were given.

Ferrari 599: 1.7 miles per gallon
Aston Martin: 2nd to run out of gas
McLaren: 3 rd to run out of gas
Lambo Murcialrgo: 4.1 mpg
Audi A8: 5 miles per gallon