318 MAX fuel economy builds?
In the mid 70's I drove a 1970 bare bones Duster 318 4 speed 2.94 gears with the tallest rear tires I could find. The engine had a factory spread bore 4 bbl 340 manifold with a thermoquad,secondary's disconnected,headers and full dual exhaust. I would drive it from Pittsburgh Pa to Denver Colorado ruffly 1600 miles.I would do it 4 times a year.I would average between 24-26 mpg running the then speed limit of 55 to 60 mph.Weight is a big factor.
as is rpm. and the type of trans.
You had the exact combo that I was referencing earlier, namely: the Hi-compression engine, in which 60mph = 2100 or so, but yours had a manual trans. Not to mention, the Duster is not as much of a brick/shoebox, as are the cars that the OP is considering. Plus your 318 had a real and decently calibrated distributor, headers, and dual exhaust.
Those hi-compression 318s were pretty darn nice; advertised at just 45hp less than a 340, but at the same maximum torque, albeit 800 rpm sooner. IIRC it was 340 ftlbs at 2400rpm. That's the kindof torque you need for a cruiser. And 26/32 is 81% of my imagined target (32.8) so you were doing pretty darn good. And that, with a factory engine! no less.
Btw,
in those years, I also had one of those hi-pressure 318s, but in a 71Demon, and it did about the same, with 2.76s and a 904 which was by far, the best-shifting trans I ever owned. So I cloned it, making 3 copies. Over 45 years later, I'm pretty sure that I still have one left.........