I have an 2006 2500 Ram 4x4 with 3,000 miles on it i have 35"tires the dealer put on it. It has a 3. 73 gear i only get 12mpg if that i average 295-350 miles on a tank. I have been told this normal until 10,000 miles & other people say i should get 20 mpg. does any one know for sure. this is my first diesel pick up.