75/76 Dodge Dart /6 average MPG and improvements

I had a 74 (?) 318 Dart 4Dr that once got 24 mpg whill traveling through Oregon and Northern CA on I-5. It was a black & white. ex City of Bellevue cop car. It had A/C, bucket seats, rubber floor mats, and no console. My wife was driving, which no doubt had something to do with it. LOL