to take an example with somewhat made up numbers.
a Ranger bi-turbo would have fuel costs of around $7 for a 50km drive.
a Ranger PHEV, if run 100% on battery, and you bought the charge at peak rate, would cost about $5 for the same 50km drive.
a Ranger PHEV, if run on 100% ICE would cost about $11 for the same drive.
that's quite a difference, but it's not real! Even if you really only drove that short a distance a day, you might find the ICE cuts in, so running 100% EV on a PHEV is pretty unreliable. IF you drive a longer distance, and deplete the battery, then the petrol engine costs more to run than the diesel.
I think the only way to truly know the costs, is to go and take a test drive. Drive your commute routes, with a number of different PHEV, and try and work out how much the engine runs, versus pure EV.
I'm not really convinced the runing costs are less than ICE, and I'm a fan
