I have a new to me 2001 Z71. 45,000 miles, 5.3/4l60E I think, 3.73 tail gears, 6" lift and 285/75-16 All terrains. The vehicle is all synthetic fluids. I just finished a road trip from Tucson to Prescott and average mileage was 11.6 mpg. There are no posted codes. Any ideas on where to start? thx matt