That depends on how it's ben taken care of, a way to find out is to trip the odometer when you fill the gas tank (do not top the tank off), next time you put gas in the tank, fill it up (do not top the tank off) divide the mileage by the gas used, do this at least 3 times to get your average mpg.
1 billion miles to the gallon
8 miles
14-20 city
I average 15.8 MPG
5
SUV's constructed by GMC offer a variety of miles per gallon ranging from the GMC Terrain that does approximately 30 miles per gallon on the highway to the large GMC Yukon that does approximately 25 miles per gallon. Also, the GMC Acadia will get around 16 to 18 mpg on day to day driving.
it gets 19.78963592647 gallons per mile
The fuel tank capacity of the 1999 GMC Sierra 1500 is 26 gallons. Its fuel economy is 13 miles per gallon in the city, and 16 miles per gallon on the highway.
11-13/city 14-17/hwy
The newest GMC Sierra 1500 gets about thirteen miles per gallon in the city. This is a fairly average mileage for most pick-up trucks, though much lower than most sedans.
i hav you can go to gmc se one it gets around 12 on the highway you can go to a gmc website and it will tel you all the specs
Click the link.