You know, one joke I've always had with my car since the day I bought it was that no matter what I do, it gets 20 mpg, period. I had a 318 with a 4 speed and low 3.something gears (I think, don't recall what was actually in the rear end when I bought it) and it got 20 mpg on the highway. Swapped to a 5.7 Hemi with a carb and kept the 4 speed originally and it got 20 mpg on the highway. Converted to fuel injection and a T56 and low and behold, 20 mpg on the highway, lol. Granted I'm probably getting twice the horsepower of the 318 now and I have half the cruising rpm I used to, but I think aerodynamics are the ultimate limit of what the car is going to do consistently. It's a brick, there's no getting around that. With better tuning I have finally managed to make that number budge a bit and have seen 25 on the highway on my instantaneous gauge readout, but it's basically low 20s.
It all kind of makes sense when you think about it though. A car needs X amount of horsepower to travel at 70 mph. Doesn't matter how you make it, that's just what it takes to overcome drag, friction, etc. You can spin an engine at 4000 rpm with almost no throttle input and make the 60 hp or whatever it takes to push you along or you can spin it at 1500 rpm with your foot halfway to the floor. Both make the same horsepower in the long run. Yeah, you might lose some extra horsepower to rotating friction and stuff at higher engine rpm, but it's likely fairly minimal. The main difference comes down to the volumetric efficiency of the engine at those different operating points. You can kind of parallel that with the torque curve of an engine, though the effects of less than WOT make dyno charts somewhat less useful for that. Generally speaking though if you have a nice flat torque curve then the engine is probably pretty efficient at any operating rpm and your actual cruising rpm may not really make a huge difference on fuel economy.
