I'd bet the MPG readout is more accurate then most give it credit for. The ECM knows exactly how much fuel is being used by how many mili-seconds the injectors are open. Injector flow rate, injectors open time and distance traveled is just simple math for a computer. The amount of fuel being put back in and speedometer accuracy are more likely the issue.
For example, New tires travel fewer revolutions per mile than worn out ones do, so the speedometer accuracy changes as your tires wear. Tires with 19/32" tread worn down to 3/32" of tread are now 1" smaller in diameter.
2014 Rubicon, 6 speed, Mopar 2" with Fox, 285's on Level 8 Punch Wheels
2015 Renegade Trailhawk