The only problem is MPGe appears to be more precise because we see several cases of identical "kWh/100mi" but different MPGe. For example:
- 31 kWh/100mi - 106 MPGe - Chevy Volt
- 31 kWh/100mi - 109 MPGe - BMW i3-REx
- 31 kWh/100mi - 110 MPGe - Honda Clarity plug-in
The Wiki article has more details:
https://en.wikipedia.org/wiki/Miles_per_gallon_gasoline_equivalent
There seems to be a consensus among the Usual Suspects posting to comment threads at the InsideEVs news site, that the term "MPGe" includes charging losses. That is, that the kWh are measured from the wall, not from what's stored onboard the car in the battery pack. Note this is directly contrary to what the Wikipedia article says about MPGe. Of course Wikipedia isn't always right, any more than any encyclopedia is always right, but when they ask you what their source is, they point to one single reference in a FAQ at a government energy agency site.
I don't know who is right, but if there is a discrepancy in the figures, that might account for it.
Given what the Wiki article says, it may be that the term "MPGe" is used to mean different things by different people or different agencies. Perhaps some mean it to be -- as the Wiki article claims -- only a measure of how efficient the energy stored onboard is, and thus about as close to the MPG metric as we can get with EVs. Perhaps others, as the Usual Suspects posting to the InsideEVs news article comments insist, mean it to be a measure of electricity drawn from the wall, which includes charger losses.
So far as I'm concerned, it's not a settled point, altho the Usual Suspects over on the news site disagree.
* * * * *
My opinion:
I have a personal distaste for the term "MPGe", not only because it's confusing -- and we're having this discussion precisely because it's confusing, in that we're not really sure exactly what's being measured -- but also because it seems to be a crutch aimed at the general public, who have been educated in the ways of gasmobiles and what "MPG" means, but not in the ways of EVs and what such things as "watts" and "kilowatt-hours" are. Also, sadly, the general public has not been educated in school about the difference between power and energy, which you would
think would be part of basic introductory science courses in elementary school, or at worst in middle school, but it's not.
As I've said elsewhere, it's as if back in the days of the Model T, the government had instituted a measurement of BHDe (Bales of Hay per Day equivalent), instead of MPG, to ease the transition from those familiar with the horse-and-buggy but not motorcars. If we had introduced that crutch back circa 1908, would we still be using it today? Unfortunately, I kinda think we would.
As they say: "Begin as you mean to go on." If the idea is to educate people in the ways of EVs rather than gasmobiles, then we shouldn't be using the crutch of "MPGe", which seems to suggest that EVs are just a kind of gasmobile which uses a special fuel.
Again, all this is just my opinion, and obviously many disagree.