Fuel Mileage on 2011 GMC Sierra

[blacksierra1]

I recently purchased a 2011 GMC Sierra 4WD, 5.3 Liter engine, auto Trans and on my first hwy trip recorded about 20 Miles per Imp. Gal which is about 17.5 miles per US gal. This seems low to me since the US fuel rating is 24 mpg Hwy and the sales sticker showed 26 mpg. Anyone have any comparables?
By the way, I was not towing and was in 2WD.

 

[Chzzzit]

I have the 2011 Sierra 3500HD 6.0 gas and it gets 12mpg highway.

 

[airwolf]

I have the 2011 Sierra 3500HD 6.0 gas and it gets 12mpg highway.

I have a 2013 2500HD with 6.0 Z-71 gas and it gets 13.4 highway, 11.9-12.2 city and 8mpg towing a 2500 lb. trailer. No mileage sticker for this vehicle.

 

[JCMELCHER]

2012 SLT 4x4 5.2L Crew Cab, I get about 16MPG City, 20MPG HWY. I did read in the owner's manaul that keeping it in 2WD versus Auto will help increase the miles per gallon, but by how much I have no idea.

 

[Grffntm]

I noticed my fuel mileage dropping shortly after that the check engine light came on, after five trips to the dealer they are rebuilding the motor due to oil leaking into the cylinders. I have lost faith in gm to build reliable vehicles.