I recently traded back in june for a 2009 silverado z71 with the 5.3l and active fuel management. I love the truck, but it is getting terrible fuel mileage. I have been averaging 13 in town and like 15 if i am lucky on the highway. My last truck with the 4.8l averaged atleast 18 on the highway without AFM. So here is my question, how do i even know if it is working, anyone know? I dont have the model that has the DIC(driver information center) that shows current mpg and such.....any ideas? I have considered putting exhaust on it so that i will atleast be able to hear it when it switches to 4 cyl mode. Just sucks when it stickers at 14 and 19, and says with reasonable driving habits can expect up to 23 mpg on the highway.