I doubt you'll see a 4WD truck get 20 mpg without keeping the speed below 60mph. There is too much drag in the front axle/transfer case. The type of tire & air pressure your running will also effect the efficiency of the truck. Running street radials at 60+ psi will do better than all terrain at 45-50 psi...that is what I'm doing Michelins street radials at 60psi.