75/76 Dodge Dart /6 average MPG and improvements

just anecdotal story, but I once got 29 mpg out of my rock stock 225 /6 automatic, 66 dart convert. I did it on a trip on I-5, from Seattle to Bellingham and back. I have to add that the car had close to 300K miles on it, and the engine was so loose, I could spin the crank by the dampener with one hand (spark plugs in! LOL)