First generation Dodge/Plymouth neon with a five speed. You start out with an epa rating of 36/37mpg highway. You have the option of basically upgrading everything with either cheap aftermarket parts or the junkyard. Of which neons always populate.
You can check out my project log
www.neons.org • View topic - My ecomodded neon. Tire spats & mini folding mirror install.
I haven't updated it in a while, but basically if you use a transmission with a 3.55 final drive/.72 fifth gear (they come on normal sohc neons), low rolling resistance tires, a grill block, OEM base model side mirror delete, removing the power from power steering, cheap under drive pulley, easy and cheap weight reduction, you're going to be incredibly close. The low rolling resistance tires are going to be the only expensive thing. You'll get over 50mpg with some light hypermiling.