List your MPG on your transplant 5.7 or 6.1

I agree with the comments regarding trans gearing and aerodynamics.
Back in the mid to late 1970s there were some tips published to help increase gas mileage. One of the main things about the earlier cars was that they were not aerodynamically efficient. The article cited the NASCAR aero car modifications that were done to overcome the box shape. It was recommended that a front air dam or chin spoiler be installed along with "cleaning up the under carriage" by fabricating a plastic or thin aluminum sheet that would help the air pass under the car more smoothly. The American cars of the time sat higher off the ground than today's cars and that resulted in a lot of turbulence collecting in the engine compartment which caused drag and thus decreased gas mileage. An overdrive transmission was also advised but the cost of doing that was not practical at the time since there weren't a lot of used ones around that would fit.
The article said that an egg shape is the ideal shape for a car to get the best gas mileage and that the compact cars would gradually assume that shape.

My 5.7 Charger has the aerodynamics and the overdrive tranny but it takes the Multidisplacement function to get 25 on the highway and 16 around town.