i just got an 04.5 2500 ctd and finally got some free time this weekend so i got around to changing my oil and filter and fuel filter. i also took my muffler off. but ive noticed my mileage has dropped from around 15 to 11 around town. i havent been on the highway much with it so i dont have any numbers for that. does anyone have an idea what might have caused this? is there maybe something i did wrong?