What mileage does your slant six powered ride get?

1968 Dart, manual steering and brakes. 904 auto. Rear? Don't remember - whatever is stock... Registration said 2700 pounds.
Had 14" tires, some type of radials back then... (early 80s)
Perfectly adjusted valve lash. Holley Economaster 1 bbl., using the original Holley jet, which was richer than the Econo. Timing was up a few degrees, barely knocking at part throttle. Heat riser worked fine. Stock exhaust.
Reading the plugs: All were "text book tan"
Got 25 city or highway. Driving easy!!!

Keep in mind that the biggest restriction in the entire system is the throttle plate!!