Until losing his election last month, former London Mayor Ken Livingstone had hoped to encourage residents to sell their high-performance sports cars and SUVs by charging a £25 (US $50) environmental tax each time they entered the city's downtown area. Drivers of politically favored hybrids like the Toyota Prius, on the other hand, would either have enjoyed either a steep discount or a free ride.

A test aired Sunday on BBC's Top Gear television program, however, casts doubt upon the notion that a hybrid would be the most fuel efficient in every circumstance.

"This is a BMW M3," the show's host Jeremy Clarkson said in introducing the car that would compete with a Prius. "It is not designed to be as economical as possible; it is designed to be fast."

Clarkson chose the most extreme examples to make the point -- a sedan equipped with a V-8 engine producing 414 horsepower against the Toyota Prius with its 76 horsepower hybrid motor. The EPA rates the BMW at 14 miles per gallon in the city, and 20 on the highway which compares unfavorably to the 48 and 45 figures for the Prius. 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."