I especially like how some people expect/demand NASA-like precision for domestic vehicle fuel consumption meters.
In most cases, a calculator will always be more accurate than fancy overseas electronic doo-dads that they put in automobiles. You can get one for $1 at the dollar store (though I'd say that they're also made overseas, too).
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
Has anyone ever noticed if jeep MPG readout is more accurate depending what season it is? Depending on what type of sensors used, temperature can greatly effect the error in what the sensor reads out. Really hot summer days or really cold winter days could effect the sensor performance. During the summer months i found that my jeep readout was roughly .7 mpg less then what I hand calculated.