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.