MPG Motor Magnum vs. Hemi

I have a 5.7 Hemi in my 2007 Charger RT. I get 15 mpg around town when I really use a light pedal. I can get 25 on the highway with cruise control and the system running on 4 cylinders.
Back in the 1970s, I was able to squeeze 18 mpg highway out of my 361 big block in a B-body by using a 2.76 gear ratio and keeping the vacuum in the "economy" range.
I agree with the guys who say the best money spent for gas mileage would be on an overdrive unit/tranny.