318 MAX fuel economy builds?

Yeah, I totally agree, there's much more to milage. Your wife's 10 mpg is what I would expect from a lead foot driver. And you getting up to 18 doesn't seem impossible under ideal conditions. But 22 MPG??

Some other factors impacting the mileage when I got my best of 19.6 mpg:
☆ As you mentioned, driver. I was driving like there was an egg between my foot and the gas pedal. And with hills and valleys, I didn't try to maintain a constant speed; I let the car speed up going down and slow down going up (i.e. avoid moving the throttle.)
☆ Also as you mentioned, tire pressure. I was running 36 psi in the 215 (f) and 235 (r) tires.
☆ Fluid type and viscosity: I have synthetic 10w-30 in the engine ATF+4 in the transmission, and synthetic 75w-90 in the rear.
☆ Aerodynamics - a 70 Dart has terrible aero, with the flat grill inset into the nose, drip rails, concave rear window, etc. But can it be any worse than a 2000 Durango?
☆ And one of the most important things: WEIGHT. My Dart is about 3150 lbs unloaded, your Durango 4600 lbs. With your truck being loaded up it was probably a solid 1800 lbs more than my car!

Did the 22 mpg calculation come from the Durango's trip computer?
nope, pen and paper. it only lasted until the high center rib wore down even with the rest of the tread. and with Wifey driving it it never repeated it either.
but it was 22 MPG both ways that trip thru 4 tanks of gas.