I wouldn't be happy with 12 mpg. The first thing is to have the diagnostic fault codes read and recorded. The people at many auto parts stores will do that for you for free.
If there are no codes and no running problems, the most likely cause is a sticking brake. Park on a slight incline, shift to neutral, release the brakes, then see if the truck creeps downhill on its own.
Monday, June 15th, 2015 AT 8:30 PM