I was getting 20-21 on the highway with a 2WD Xterra. Now I have my 4x4 and on the first tank I got 18.9 mpg calculated by hand. That is driving 70 on highway with about 50/50 highway/city driving.

I accelerate like i have a tack in my toe. I go easy on the acceleration. Biggest things that affect mileage are driving habit, tire pressure, wind/weather and terrain.