Volkswagen Auto2000
The 1981 Volkswagen Auto 2000 obtained 63 mpg city, 71 mpg highway, boasting a 0.25 drag coefficient, 53 horsepower diesel engine, and 1,716 lb curb weight[1].
Volkswagen's E80 diesel concept obtained even better fuel economy. Using a 51 horsepower supercharged 3-cylinder turbo diesel, the 1,540 lb Volkswagen E80 managed to obtain 74 mpg city and 99 mpg highway[1]. It had a 0.35 drag coefficient[1].
References:
[1] Bleviss, D. (1988). The New Oil Crisis and Fuel Economy Technologies. New York: Quorum Books