318 MAX fuel economy builds?

I have no idea how some of you guys are getting 20+ mpg on an old carbureted V8. I'm measuring my MPG super closely, and I have to wonder if everyone else is doing the same. I fill up the car, log every mile I drive, fill it up again, and calculate accordingly based on distance driven and fuel used. I have been doing this for every tank for 25+ years, initially because the fuel gauge didn't work and now just out of habit. So I'm darn sure my MPG calculations are as accurate as possible.

My current setup is this:

Stock heads, standard bore 5.9 Magnum. Engine doesn't burn any oil or smoke at all.
Factory EFI, factory tune (1994 OBD1 PCM). Injectors are clean, fuel atomization is perfect.
Mopar M1 EFI intake
Hughes SER0814ALN-14 roller cam with their Magnum springs
Comp roller rockers
46rh transmission with lockup converter.
Doug's long tube headers into 2.5" exhaust.
Electric fan
3.23 gear

With my 26.5 inch tire I'm spinning about 1700 rpm at 60 mph, and the best I have ever gotten on at 80 mile highway trip is 19.6 mpg. Admittedly that was at about 70 mph, and that highway has some hills so it would be better on flat ground, but that was me TRYING to make MPG (careful throttle modulation, don't stomp on it, etc).

Before the 46rh I had a 904 with a ~2200 rpm converter, and the absolute best I ever got was 16 mpg. That was taking it super easy on country roads running around 55 mph. On the highway at 65 mph I would get a little above 15 mpg, so I guess the 46rh giving me +4.5 mpg on the highway is pretty good.

I'm sure the stock cam would be better for MPG, but I don't think a lot better. My cam doesn't have a ton of duration since it's designed to work with the factory tune. Also, I went from the stock kegger intake to the M1 intake back when I still had a 904 trans, and I actually got BETTER highway MPG. The 904 trans + 3.23 gear was turning 2600 - 2700 rpm on the highway. I think going back to the stock intake with the OD would probably be better for MPG; that kegger is a low end torque monster, but dies around 4500 rpm.

Before the EFI 5.9 I was running a slightly warmed over 318, Eddy 1406, 904 trans, factory converter, and 3.23 gears. The absolute best I ever got was 15 mpg on an 80 mile highway trip, running at 60 mph or less the whole way. Usually that setup got me 14 mpg on the highway, 13 mpg around town / mixed driving, and 11 - 12 mpg if I had my foot into it a lot.