Flexfuel Dave, I have exactly what you have but in 4x2. The best I ever did was just under 21 in lots of highway driving. My tires are always well inflated and I'm gentle on the gas pedal. Shouldn't your 4x4 use more than the 4x2? It sounds like it doesn't make that much difference. Is the heater a big reason for worse mileage in the cold?