Slant Six mpg's?

FWIW, my Dart Lite got a consistent 18 with a combination 25/75 combination of in town / and highway, and the highway runs were 55-65 in the AM, minimal traffic, and 25-30 average in the PM in "stop and go" traffic. Combo is 3 speed with 2.94 gears on modern radial tires, and junk carb, overall. 55mph was happy, but keeping with traffic had me at 65 which turned more RPM then I liked. With the OD trans, better carb, and mpg inspired tweaks (belly pan from bumper to K member, HEI mod, fresh tune up, index plugs, synthetic fluid in trans and diff) I am shooting for 25 combined.