Re: 4.6L vs 5.4L
Light weight aluminum rims, aluminum driveshaft, remove AC, electric fan...
I think the 4.6L gets about 19mpg, 5.4L gets about 16mpg. Best way to add mpg is to reduce rotating mass. 10lbs of rotating mass=100lbs of non-rotating for mpg increase or so. Also, changing the rear gearing can help. Higher gearing for lots of stop and go, lower gearing for highway.
“Adults are just obsolete children and the hell with them.” - Dr. Suess