I cruise along on the highway at 65 (speed limit). We have some 80mph zones between here and Lovelock on I-80 - on that I do 70 - even the semis that have any brains do 70. I get 100 yards behind one of them and just stay there.
Lots of people with out of state plates wizz by at better than 90.
It's amazing what a difference it makes when there is a headwind or tailwind:
Level freeway at 65
No wind = 21 to 22
Strong headwind = maybe 19
Strong tailwind = up to 24
Mind you I have nothing modified on this wagon except for the tires which I keep at 40 psig.
No lift. no roof rack, no heavy bumpers, no body mods. Heaviest loads generally about 650 pounds (me, the wife, off-road gear<tow strap, d-rings, hitch without ball, fire extinguisher, a couple blocks of wood, first aid, 6 gallons water, shovel, hand winch, axe, tire pump, small box of tools - includes multimeter - , a few towels/rags, box of topo maps, OBD2 scanner and list of codes, rope, gloves, extra coats, extra hats, t-paper, gallon of washer fluid> up front gps,small camera, lunch and two thermoses of coffee) Nothing is carried externally.
Rarely on a really long trek I'll add 10 gallons of fuel (in two old fashioned steel gerry cans), sleeping bags, food, etc.
A couple other thoughts: I do on very rare occasions air down tires (can't remember last time I did it. I have never owned a mounted winch, sand ladders, extra armour, external lights (what came on the car is more than sufficiant), nothing dangling from back bumper.