MPG Motor Magnum vs. Hemi

I've owned tons of Magnum and Hemi powered trucks and vans. Mileage ranged from abysmal to horrifying. My wife has owned a few Hemi cars and they all got mid-high 20's on the highway. All of those were 6 speeds though. I suspect the trans is the reason. Never owned a Magnum powered car (don't even know if they made one).