I believe they also were rating the original Omnis at 48 or 50 mpg back then. It wasn't realistic.
Given the weight, aerodynamics, and gearing (and taking into account engine power), I'd say a realistic max is 28 mpg.
I'd expect a slant six to get 15/22 with an auto, 14/24 with a 3-speed and 15/28 with an OD 4-speed and the tall gears.
I have a 2004 Chevrolet Colorado 3.5L/5-speed stick 2WD and 3.42 gears and if you're really nice to that it will get 27-28 highway. Its probably more aerodynamic than an A-body, but is a bit heavier.
