I suspect its just a matter of Gallons left * Average MPG over a period of time. The question is what time period and for how long. The fluctuations in your average MPG will account for the fact that it doesn't always go down by a mile for every mile traveled. The only way it would is if you CURRENT MPG over that mile EXACTLY matched the average MPG. Your current MPG over each mile you drive fluctuates rather wildly unless you are at constant speed alone on a perfectly flat road as those of us with a Scanguage, NAV or instant MPG will attest.

Add in a ~50 MILE buffer at the end to make sure you don't run out of gas (actually subtract that) and you are done.

Yeah... you can pay attention to the SOC of the battery but if you can only go for a couple of miles on that battery alone, its "help" even when you are driving with the ICE on will only account for a few miles as well added on. I just don't think SOC is worth it and I doubt they bothered. It doesn't have to be that accurate.... as long as it always reads 0 BEFORE you run out of gas and reasonably close to the end of the tank, its going to be considered good enough.

 

Think about this....

You have been driving fairly aggressively, FAST, but suddenly you notice that the indication of the MTE will not allow you to get to the next "fuel stop". So you slow down in order to get a better FE and maybe reach that fuel stop after all.

So, does the on board computer now use the "new" short term average of 20 MPG, or the previous 15 MPG.

In the short term it will still use 15 MPG, and now the longer you maintain the lower speed, 20 MPG FE, the more the calculation will "lean" toward the better FE.