I've gotten as high as 23 mpg with my Ranger (4.0 ltr V-6, 5 Spd manual, supercab) Kept the speed down at 65, all highway, cool enough to not need AC. That's a long way from 84 mpg. Usually I keep up with traffic and the mileage drops to ~22 mpg.

I suspect he's running the short bed, standard cab, 4 cyl, 5 spd and keeping the speed low enough that drivers behind him are pissed.

That said, there's a point around 50 MPH where it becomes more efficient to roll up the windows and turn on the AC.

Getting tire pressure near max always helps. I run mine at 38 psi cold, max is 44 psi. I'd up that to 40 psi, but the ride gets pretty hard.