199.9 or True Average for Total MPG Figuring?

If this is a stupid question, please forgive me, but I was wondering... when the car shows the TOTAL MPG from Day 1 (as seen next to the normal Odometer reading), how accurate is it if you've driven a lot of EV miles? For that figure, does the car use "199.9MPG" for the all-EV stretches, or does it instead figure out the total gallons used ever and compare it to the total miles driven? Because "199.9" for EV miles isn't accurate at all for all-EV stretches (you could argue you're getting 1,000,000+ MPG since no gallons of gas is being used -- and MPG is miles per GALLON).

So let's use this example of driving 1,200 miles (with nice easy figures):

* You just bought your brand new Prime. It has 0 miles on it.
* Your first 600 miles are all on gas (no EV), and you manage to get exactly 60 MPG, meaning you've used exactly 10 gallons of gas for those 600 miles.
* Your next 600 miles are going to be all with EV (no gas used at all).

So now you have 1,200 miles on your odometer.

But my question is... when the Prime displays your TOTAL MPG for the life of the car (1,200 at that point), will it show:

(a) 120.0 MPG (10 total gallons used for the 1,200 total miles you've actually driven)
or
(b) some other figure that it gets by adding "60 MPG" for the first 600 miles, and using "199.9 MPG" for all-EV 2nd 600 miles?

But what if someone instead drove 99% of those 1,200 miles on EV and only 1% using gas? The display would still only show 199.9MPG, when it should be much higher.

So is it just the DISPLAY that has the 199.9 MPG max limit, or is the car's BRAIN actually using "199.9 MPG" for EV stretches (when figuring out mixed Total MPG for the car)?

 

Thanks for the answers folks, good to know!
I've hardly been driving lately (work's slow, and it's the summer) but in a way, having an accurate "Total" MPG (from Day One) keeps driver's honest. On my old Honda, it only had MPG for "Trip A" and "Trip B" which you could reset at anytime, so one could floor it and drive terribly, then reset it as if it never happened. But the Prime remembers all!

 

I suppose, though work to pay the bills (or to save for a "real" vacation) would be nice. Like most people, without realizing it after a while I rarely bother with what's in my own backyard anymore. People come from all over the world to go to the local beaches nearby, but I never go unless we have out-of-town company visiting (which means I basically go about once every 4-5 years now).
Actually SoCal is relatively pleasant in June, July, and August. Instead, we get our worst weather in Sept and Oct, when the very hot, dry Santa Ana wind conditions come (along with all the fires that come with it).

Well, I do have some work on Saturday -- and this time for a change, it's (barely) within the EV range of the Prime... so time to charge it up Friday night!