If it were me (my Rubicon 4xe is not built/delivered yet), I'd look at "fuel economy" with the following:
1kwh = .03 US-gal in terms of energy measure / 1US-gal = 33.4kwh
Since 15% of the 17.3kwh battery is in reserve, it takes ~15kwh to charge (ignoring battery charging inefficiency here). That is equal, energy-wise, to .45 gallons. If one goes 21 miles on 15kw, that yields almost 47mpg"e" which is pretty close to the EPA "e" rating. Looking at total miles driven and adding how many full charges you make (from "<1%") can tell you how much energy you use and using, the ratios, figure out your true "mpg". My guess is that the on board computer is going to be very close.
Example (I recall a video but can't find it now):
One full-tank and starting on a full charge, their Rubicon travelled 390+/- miles.
Assume that 15kwh was used to charge, so that's equal to .45 US-gal and then they filled up with almost 15+ gallons so I'll round-up to 16 gallons and add the .45 "gallon" from the battery; that yields 23.7mpg. I believe their 4xe trip computer stated 24mpg.
Hope this helps those trying to see how efficient/inefficient their Jeeps are running. I can't wait to do some controlled, scientific tests when I get mine.