how should a consumer measure mpg?
How should a ev/phev owner measure mpkwhr?
For a PHEV owner driving exclusively on plug-in charge, something like this
measures the electricity consumed by charging: https://www.amazon.com/P3-P4400-Elec..._df_B00009MDBU
Your odometer measures miles (if you trust it, otherwise, look for a highway that marks off measured miles).
And then the math for "mpkwhr" is just the miles divided by the kwh reported by the meter.
For mpg, the usual drill is to fill the tank, reset the odometer, drive a good distance, refill the tank again (preferably at the same pump and in the same way), and then calculate miles driven divided by gallons added (on the second visit).
Regarding "the same way" to fill the tank: a lot of people seem to like to try to squeeze in a bit more after the pump first kicks off to indicate a full tank. When you do that, you introduce inconsistencies in your technique (which will affect your mpg calculation), and you also expose yourself to some potential problems. It's best to stop when the pump kicks off the first time. If you overfill the tank, and then it has a chance to warm up and expand before you've consumed that additional amount of fuel, you're probably either over pressurizing your fuel system or else pumping raw fuel (rather than just vapor) into the evaporation emissions control system. In some locales, due to vapor recovery technology on the gas pump, you might also wind up with a big spill on the ground if you try to squeeze in a bit more after the pump first kicks off. I learned that lesson the hard way, one of the first times that I tried to "squeeze in a bit more" from a gas pump in California. Never again.