I think if you had actually started that 5.9, you'd have seen mileage in the low teens. Don't know where that 20 mpg comes from, but I don't think I buy it. My '03 averages 13, but that's 'cause it's almost all highway. If I drove it in town only, I suspect it would be between 11 and 12.

 

In mixed driving, my 5.7L 2wd is averaging between 17.5 and 18 right now. 60%hwy, 40% city. All city about 15.