Gas mileage will probably average 13-15, or less...depends on how much you keep your foot out of it. You can easily burn up a tank of gas pretty quick, but if you stay out of the boost you'll do decent on gas. It's a heavy truck with a lot of power, two things that add up to terrible gas mileage.
In 2003 the model year changes included, revised cylinder heads with more threads added for spark plugs, new engine block with thicker main webbing than previous years. Payload increased to 1,350 lbs from 800 lbs.(rear sits higher). There is a new design for the wheels, a change in the fog light design to prevent internal cracking. The 6-disc remote CD changer was replaced with an in-dash 6-disc CD player. Brushed finish was added to interior door handles and buttons. The storage/tool box behind the seats were deleted and passenger seat child security buckles were added.