Small block mileage

I had a 74 Duster with a 318 2 barrel, 72 340 exhaust manifolds with dual exhaust to the rear bumper that got 23 mpg on the highway. My 70 Challenger 340 used to get 20 mpg on the highway, but I had swapped 2.76 gears into it because I had a year of road tripping to do between Asheville, NC and Clarksville, TN. My 71 Challenger conv with a 360/727 and 3.23 Sure Grip got 20 mpg on a trip to FL and back a few years ago. The convertible really surprised me doing that well. It has a MP cam, LD340 with an Edelbrock 600 and 70 340 HP manifolds with 2 1/2 inch exhaust.