Plus the 68 didn't have the Rube Goldberg smog contraptions Detroit was using customers to beta test for them.I had a 1968 Dart 225 908 auto. After putting on a Holley Economaster, and jetting one step rich, I'd get 25 highway or city in good weather.
Also had radial tires on 14x 5.5 steel wheels.
The car weighed in at an honest 2750 lbs. Bench seat. AM radio. Manual brakes and steering.
Was a nice car for the $600. I paid. Had 95,xxx miles on it when purchased. Also, lashed the valves - huge improvement in power!
I had it for about 3 years in the early 80s.
Bottom line: you don't always need a manual trans for decent mileage.
Later, I had a 74 Dart, 198, 3 on the tree. I could not top that 68 for power of MPGs. The 74 weighted a lot more! (400+lbs.)
Power steering and brakes, door side beams, bigger bumpers.... All adds up