I think the trick to getting 20 MPG is having 6 speed manual transmission, 2 wheel drive,highway treaded tires and drive like a little old lady.
I get 19 - 21 all the time on my 99 F250, 6 speed manual, 2 wheel drive (4X2), highway tires, synthetic oil, flat east coast highway driving. And yes I drive like a little old lady.
That might change soon, adding gauges, exhaust, DIY Air and TW chip soon. From reading about everyone else's mods, may be hard to keep driving like a little old lady after the mods.