i bought a used 2014 1500 Big Horn with a 5.7 hemi and I use 87 octane gas in it. My dash computer shows the gas mileage as about 10% better than what I calculate when I divide my miles driven by the number of gallons burned. For example, my "Trip A" readout that was reset when I last filled the tank said I had averaged 19.6 MPG. My hand calculation for the same tank of gas showed that I averaged 17.6 MPG. This has been pretty consistent and the truck always reports better mileage than what I'm actually getting - never worse.
How accurate is your mileage calculator?