Into the weeds engine design for fuel efficiency discussion.

Dale Davies said
A friend from years ago bought a Super Bee that a previos owner had installed a 426 Hemi into. They had also installed a Corvette IRS with the stock 3.07:1 gearing. On the highway that car would get 21.3 MPG US. Suppose the trans was updated with an OD and dual throttle body sequential injection was installed. Lots of air flow with excellent fuel management. I can envision 25+ MPG.
The car still had the drum brakes on the front with the Corvette discs on the rear. Really needed the front brakes changed to discs.

Highway driving is easier to increase if that's where you spend major of time clock at lot of miles a year gearing lockup OD etc... could be worth while improvements, intown is a different story unless it's little encono box most cars i've driven gets mpg in the teens my Avenger RT gets 19-22 mpg probably one of the better fuel mileage cars I've had.