If I lived in Colorado especially, I would only want a diesel. It is not the mileage but the longevity and power. Engine braking is another major factor. Even with mountain driving, I rarely touch my brakes. If you already have or plan to buy a 250/2500, you should be able to make it work. Supersprings and upgraded shocks will do a lot for little money. I did that until I replaced my truck. My 3500 is basically the same truck with a bit more stiffness in the rear springs. I have not needed Supersprings or upgraded shocks. I did add Timbrens to level the ride and to keep my headlights from blinding oncoming drivers.
