The mere fact that in HV mode the car is deciding at any particular point in time whether it is using battery only or gas-assisted electricity makes it impossible to accurately separate the two. The only thing you can do is to agree to accept that if you drive 200 miles in HV mode, and you used 5 gallons of gas, you averaged 40 mpg even though some of those miles may have been on battery alone, even in HV mode. It could be that 12 of those miles were handled by battery alone, without the engine running. In that case, you only got 37.6 actual mpg on those 5 gallons. You simply don't know and there's no way to find out without staring at the power meter constantly and manually jotting down info instead of paying attention to your driving.
We will have to agree to disagree on this one. If we carry your arguement to absurdity, you could say that none of your miles are gasoline (because the Clarity is fundamentally a serial hybrid, and thus all the miles come from electricity, not gas). An exception of course, is the direct drive mode which departs from the serial hybrid paradigm.
Let's consider your example a little more carefully however. Let's say that you drive 200 miles in HV mode, and use 5 gallons of gasoline. I contend that you
CAN precisely state that your averaged 40 mpg
(as long as the SOC is the same before and after the experiment). During those 200 miles, it could very well have been that 12 of those miles were handled by the battery alone, but that does not change the fact that you really, really did get 40 mpg during that trip
(as long as the SOC is the same before and after the experiment).
Now, if you want to address the fact that the SOC is not perfectly maintained, then I agree... Unless you account for the change in SOC, then your 40 mpg is wrong. Let's create an example that includes this effect - You start out with 50% SOC, and you drive 200 miles (using 5 gallons of gas). When you are done, the SOC has dropped to 30%. Now, I will agree with you that it is not possible to 'perfectly' know your gas mpg because indeed some of the 200 miles were taken from the battery (which was charged from the wall earlier). But, it is easy to get a very good estimate. Roughly speaking, you get 50 miles with a full charge. This means that the 20% SOC 'droop' that you experienced is equivalent to ~10 miles (20% of 50 miles). This means that you really only got 190 miles with 5 gallons, or 38 mpg.
If your EV range happend to drift up (as you experienced yesterday), then it would mean that you mpg was actually better. If the SOC were to increase by 20% during the 200 mile experiment, then you effectively got 210 miles with your 5 gallons, or 4 mpg. The only time I have seen the EV range drift up (while in HV mode) is during a sustained downhill where there is sustained regen. Eventually it comes back in my experience.
I actually think we actually agree more than we disagree. Yes, you can't know what your exact mpg is when in EV mode
(unless the SOC is the exactly the same before and after the experiment). But you can get extremely close by including the change in SOC as an adjustment to your calculation.
