Is this a case of man bites dog? A mid-size BMW sedan equipped with a diesel engine returned better fuel economy on a 545-mile, London-to-Geneva, run than a gasoline-powered Toyota Prius. A BMW 520d with a 2.0-liter diesel engine and regenerative braking (but no heavy duty hybrid features) got 41.9 mpg while the full-hybrid Prius, 500 pounds lighter, got 40.1 mpg.

Two writers for the Sunday Times (London) wondered if official mpg ratings were accurate after hearing from their readers who said the Prius ratings were high by 15 (U.K.) mpg. "If our readers are right and the official figure is wrong it has important implications, not least of which is that people driving frugal diesels are getting a raw deal," the paper said, since hybrids get some tax and roadway access benefits in Europe as they do in the U.S. To find out, they drove BMW's midsize 5 Series sedan and the Prius London-to-Geneva, adding 100 miles to the route "to give the Prius the advantage of running in urban conditions where its petrol-electric drivetrain comes into its own."

Whilst the BMW "doesn't have the external look of a green car and you don't get the same self-righteous glow when you are driving it," it does have a series of energy-saving features BMW calls EfficientDynamics: battery recharge when braking, good aerodynamics, low rolling resistance, a continuous fuel consumption gauge on the dashboard, and a six-speed manual transmission that is rated as improving fuel economy by about 10%. Testers found the 520d "not startlingly quick ... [but] it cruised happily at the French autoroute limit (dry conditions) of 78mph towards the champagne region." When the trip was done, the BMW still had a third of its tank in reserve.

The Prius, in comparison, ran out of gas. Not because of lousy mileage but because the tank is smaller (and the testers had a spare can of gas aboard). Acceleration was "hardly dragstrip quick and slower than the BMW, but still respectable." The Prius weighed 573 pounds less and had an aerodynamic shape "as slippery as a campaign manager discussing political donations," leading the testers to believe "the Prius should take less energy than the BMW to maintain a constant cruising speed." Not so. As many people know, the advantage of hybrids lies in urban driving where the stop-and-go recaptures the car's energy. On the highway, it's one more four-cylinder car pushing throught the wind.

The full story is on the website of the Sunday Times (London): "Toyota Prius proves a gas guzzler in a race with the BMW 520d: The Toyota hybrid is hailed as an eco-paragon, so how does it fare against a big BMW? To find out our correspondents go on a run to Geneva." An Imperial gallon is 1.2 U.S. gallons, or a U.S. gallon is 0.83 Imperial gallons. Roughly speaking, discount the Times numbers by one-fifth: a 50 mpg vehicle to them is a 40 mpg vehicle to us Yanks. Another reason we should all go metric.