High Mileage replied to Brad062991's topic in 2000-2006 & 2007-2014 Chevrolet Silverado & GMC Sierra 2500HD/3500HDSadly the same corrected 14 mpg on each tank of mixed driving. 2011 2500HD, 6.0, 4x4 RCLB, 3.73, 6 spd auto. Do check you speedometer against your GPS or smartphone speedometer ap and correct your calculations. I found my speedometer to be reading a little faster than the actual speed and or distance traveled using ten and twenty mile markers on the highway compared to my odometer. And I do say "sadly" because my previous truck did 12 mpg and I didn't mind that from a 2006 2500HD 8.1, 4x4 CCLB, 4.11, 6 spd ally. With the 6.0 s' technology and lighter 6 spd tranny I really did expect better fuel economy than it provides.
High Mileage replied to 022500HD's topic in 2000-2006 & 2007-2014 Chevrolet Silverado & GMC Sierra 2500HD/3500HDToo little info to provide a guess. But it sounds as if you have a repeatable failure and that is actually good. A problem that is frequently observed lets you test and narrow down on the cause. I'd agree with servnantsalesmen on the throtlebody and more specifically the Idle air control valve for cleaning.