I'm looking at getting a used 2010 GMC sierra 1500 ext. cab 4x4 truck. It has 26 000 km and a local dealer took it on a trade. The fellow that traded it in was an older fellow who wanted to move up to a duramax diesel for his business. Truck is in good shape. It has a 5.3 engine and 6 speed auto. What type of fuel economy does this combo get? Thanks in advance.