Best way to improve mpg. Go back to stock suspension, tires, gearing, etc.

I got my best highway mpg on a round trip to Houston. 21.6 mpg, while it was new to me. About 1200 mi each way. Of course, those long hauls also bring out the best in mpg. Can't touch that today. My F350 gets about the same mpg as my XJ. Around 16 mpg, on a good day. But then, the XJ doesn't exactly get used as a highway cruiser, anymore.