Just to throw it out there, I live in the rust belt and these are all still in use.
The 1975 D300 Dumptruck has just over 100,000 miles on it and it averages 14-16mpg (with 4.88 gears, a 318 and a 4 Speed)
The Orange 1984 Dodge D50 has over 600,000 miles on her and she averages about 17mpg...