Slant Six mpg's?

My '76 Dart 225 super six, 904,2.76,215/70/14's averages 18/20. My '65 Valiant 170,904,3.23,185/70/13 does 20+. Picked it up late last summer, so don't have a real good handle on it's MPG. The day I have to worry about gas mileage, I sell the cars and start collecting stamps. AND I love the looks and stories from people who had a slantly in their past.