In the run up to the Detroit Auto Show, Mercedes-Benz decided to show off the fuel efficiency of the new Smart ForTwo and the Mercedes E320 BlueTec. They ran a fleet consisting of gas- and diesel-engined Smarts and the E320 diesel over a 2,700-mile route from Los Angeles to Detroit. Along the way, the gas-engined Smart that has just gone on sale here in the US averaged 49mpg. The diesel-engined version, which isn't yet available on our shores, managed 60.3mpg. Even the big E320 sedan managed a very respectable 40.6mpg.