In this test, the M3 matched the speed of the Prius as the hybrid ran flat-out over ten laps of the 1.8 mile Top Gear Test Track in Surrey, England. Measurements taken after the run show that the Prius returned just 14.3 miles to the US gallon, while the BMW had 12 percent better fuel economy at 16.1 miles per gallon.

"It was one of the dullest drives of my life, but in the interest of science I stuck with it," Clarkson said. "Seriously, what I'm saying is, it isn't what you drive that matters, it's how you drive it. That is everything."