Need help..have a 2008 2500HD LMM and is garage kept. Not used for work and maintained diligently. Recently I discovered my fuel mileage dropped from 14-15 mpg to 10 mpg in city driving. Very little towing, but when I do, performs well. Truck has 103k miles. Couple months ago had injectors cleaned (all injectors within specs), new timing belt, new air filter, cleaned sensor, and change fuel filter when indicated. No codes. Engine runs smoothly with no apparent issues, good power production but the DIC shows loss of mileage. Anyone have any ideas?