The best way to determine actual mpg is to reset the trip odometer at each fill up. Then calculate gallons used per miles driven with ICE only. The trip odometer splits electric and gas power modes rather well.Be careful, it's rated at 49 mpge , not mpg. Mpge is a measurement of efficiency for electrical use.
Rather than reading "49 mpge", what you should really read is "1.45 mpkWh". Mpge is a travesty of a unit.
The 4xe is a phev, so if you hop in and do a 300 mile trip without messing with drive modes, it'll basically do the first 20-25 miles in all electric, using 0 gas. After that is up, the gas engine will fire up, and you'll get 20ish mpg from that point forward.
If you ask me what mpg I get, the answer I'll give is about 22mpg, 37mpg, and about 70mpg.
When the gas engine is running, I get about 22mpg. If I go off what the dash says, it says about 37 mpg, because it tries (and does a poor job) of factoring in an equivalent mpg value for the electricity used. If I look simply at the gallons of gas put in and divide by the total miles driven, I get around 70 mpg.
Ultimately, all those values are basically worthless as they reflect more of what kind of drives I do rather than the actual efficiency of the vehicle.