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Recently finished a full tank of gas running exclusively in HV mode; got 50.7 mpg which I think is great for this car. The battery always had some charge from me plugging in each night, so going to try HV again without any battery charge to see what happens.
If you were plugging in each night then the 50 mpg is artificially high, although by how much you won't know until you run a tank in HV without plugging in. But even when you do that just one test won't be definitive because there will likely be other variables between the two tanks. But it should give you some idea.
In HV mode the battery is still used but of course gets recharged by ICE, but the battery level fluctuates by a few percent while you are driving. Anytime you are in HV there is a set point, sometimes called the target SOC which the system is trying to maintain, it uses the battery as needed until it drops a few percent below the set point then it charges it back up to the set point, and the cycle continues. When you turn off the car it will randomly be at some SOC which could be at the set point but more likely will be a little below. Then when you start the car again the current SOC will be the new set point. The same thing happens by the way when turning off HV while driving then immediately turning it back on again. This is known as HV reset and is used by some people on purpose to reduce engine noise especially when driving in the mountains.
For example if you leave the house at 100% then stop at the grocery store and turn off the car it might be at say 98% SOC. Then when you turn on the car 98% will be the new set point. When you get home and turn off the car it might randomly be at 97%. Since you are plugging then it will charge it from 97% to 100%. That's just an example it might be less than that usually, but over a tankful it will add up.
So even though you drove in HV for that tank you definitely used some grid electricity also, but there is no way to know how much unless you had uses the app to check the SOC level each time you arrived home, or do the test you are planning where you don't charge at all for a tank. Normally that type of test is done by filling up when you have 0 EV range, that way you don't have to constantly remember to press the HV button