GM Precept - 80 mpg back in 2000.
The abandoned Precept concept was created for the PNGV which was an alliance between GM and the US Government. The Precept was a working hybrid prototype propelled by a 35kW three-phase electric motor to the front wheels, a 1.3L, 3-cylinder diesel driving the rears with NiMH power storage. It was capable of 80 + mpg on the 85 - 07 EPA test cycles. The aerodynamics alone (Cd of .163) equates to a composite highway/city number increase of 12 mpg before the advanced drivetrain was taken into account.