Here is a car that wouldn't be to difficult to replicate using vw parts.
The Covington Tiburon
Its rear engine like a vw. Gets 50mpg. The car was built in Florida in the early 60s.
Has a lot of similarities to the vw based Sterling kit car.
Link to more info about the car
Covington Tiburon
If you used a geo metro xfi engine or small diesel engine instead of the 50s era engine it should get 100+ mpg.