Interesting question - I leave mine in 2hi - have a 2011 with the 5.3L V8 - what engine do you have and what mpg are you getting? I'm currently averaging 18.4 with around town mixed with short highway. 20.8 to 21.1 on hwy at average 72mph
I have the 5.3 L Fle Fuel engine on my 2011 Z71 Silverado and I get nowhere near that kind of mileage, especially in winter!
Aroound town and short highway i would say I average 15 to 15.4 mpg.
