do you suppose that part of the problem could be that with the double OD and 3.55's the final ratio might actually be too low and the tune can't put enough timing in to take advantage of the lower RPM and be efficient?
Possible I suppose but I’d think that’s unlikely. I don’t know what cam or tune he has, but the OE gearing for a 5.7L Challenger would set a 70 mph cruise at about 1,600 rpm. That’s with a .50 6th, tires that are almost 29” tall (20’s) and a 3.9 rear gear as equipped from the factory. And of course that’s a nearly 5k lb car.