I am the new owner of a 2008 Ram 2500 6. 7, the overhead readout is showing 11. 7 mpg avg. The truck is all stock(exhaust,intake,computer chip,etc. ) but the tires are 285/70/17 and originally came with 245/75/17, is this a false reading, and if so how do I reset it short of buying a tuner. I...