18.4 mpg!!!

Good morning folks,
In July I made a 4,000 mile trip and averaged between 17-17.5 for the trip. This weekend we took the Duster to Statesboro for a birthday party. Drove down on the interstate doing 70-75 the whole way (about 150 miles) and averaged 16 mpg. On the way home yesterday we came the back roads and kept the speed between 50-55 and averaged 18.4 mpg! No a/c either way.
C

'72 Duster, 318 w/A998-999 auto, 2.90 suregrip