The calculator in the 4XE has a crazy imagination and a bit of a sense of humor. It also has no way to measure the energy used when in electric mode. It calculates the miles but not the electrons.
it starts your calculations based on where your overall average is. So if your overall average is 30MPG, it sort of starts there. It figures 99mpg while driving in electric. Somehow, it weighs that 99mpg against your overall average. The more electric miles you cover, the higher that overall weighted average is.
I was messing around one night as I drove in electric mode. I kept resetting both trip odometers and the overall average MPG over and over. I eventually got my overall average to read 99mpg and that’s where it caps out.
2nd gen volt had a great way of handling this, it calculated MPGe or MPG Equivalent while on battery separate from the MPG on gas, displayed both then also displayed the average of the two together. Didn't treat the electricity as free like the 4xe display does. Jeep: this would be a nice feature add for our 4xe's in uconnect hybrid screen!
Example below (sorry for the crappy fingerprints) but you can see MPGe and MPG along with equivelant energy consumed on the right, then combined and lifetime averages on the left.
33.7 kWh is considered to be the amount of energy stored in a gallon of gasoline. Your maths then become MPGe = mile driven * 33.7 / kWh consumed
(I stole this from here: Miles per gallon gasoline equivalent - Wikipedia)
Not sure what formula the volt was using above because the math doesn’t line up with the display, but it may also not be accurate because it may be hard to track actual kWh consumed as directly rated to EV mileage when blending both simultaneous gas and electric usage. (The picture above is me going through the mountains so i was forcing the electric motor to help the ICE heavily) i suspect this will be an issue with the 4xe even more since hybrid mode heavily blends electric and ICE together at freeway speeds and the EV miles tracked seems really weird to me at that point.
But the formula should get you close if the ICE never starts and you’ve been on pure Electric. We’re rated at 49MPGe so be curious where the forum members efficiency falls in.