About all you can do is keep track of how much you are charging, either with an EVSE that tracks charging time and amounts, or by manually logging it. Then when filling up with gas multiply the number of gallons of gas used by estimated MPG to determine the estimated number of miles that were driven on gas for the previous session. Then subtract that number from the odometer distance to determine the estimated number of miles that were driven EV. With all of that you can come with an MPGe estimate or miles per kWh which should at least be ballpark. Too much trouble for me, but some people have done this for one or two gas fillups to get an idea of consumption. Gasoline MPG can be determined by starting with 0 EV miles and a full tank of gas, then drive until the tank is 1/4 or whatever then fill up to determine how much gas was used for the number of miles. People who have done this usually say they get pretty close to the published mpg of 42, so without testing for yourself that is a reasonable number to use in estimates.
Volt really set the standard on how a PHEV should operate, too bad Honda didn't follow their lead. I am just more resigned to it, I enjoy all of the pluses of the car, and ignore or work around the quirks. I drove a Prius for eighteen years (two different ones) and meticulously tracked fuel consumption and mileage, I was disappointed at first that this really wasn't feasible with the Clarity, but in some ways it feels liberating not tracking things anymore and just driving the car. I rarely buy gasoline and have decided that's enough data for me
