Chrysler could get more efficiency out of the Hemi if they wanted to. With dual plug technology and the superior combustion characteristics of the hemispherical combustion chamber, they could set up a lean-burn mode for cruising that should increase mileage considerably.
Chrysler engines used to get better mileage with carburetors than the newest EFI versions! My buddy had an old '85 Ramcharger with a 318 2bbl, and with 3.55 gears and no overdrive, it could manage a consistent 18 MPG on the highway. Power wasn't all that great, but it got remarkable fuel mileage for what it was. Same goes for the old dinosaur Slant 6. In my buddy's Dodge Mirada, 25 highway MPG was easily attainable. Again, with no overdrive. Emissions tests showed it was cleaner by more than twice as much compared to a '89 Dodge Omni 2.2 that got sniffed right before it.
The thing couldn't pull a pregnant mouse off an anthill...85 HP doesn't get you far in a car that heavy, but it was quite the fuel miser, and I don't know of anyone that ever broke a Slant 6.