I have a BMW PHEV, so much depends on your use cycle and whether or not you charge it. I have cheap electricity from 12:30am to 4:30am for charging every night. If I just do the school run and a trip to the shops in it I would never need petrol. But in the winter I can't get to the office and back on electricity only which is a shame. If I never charge it it works as a normal hybrid and I get low 40's UK MPG, which is still decent for a 225hp 4wd car. If you don't charge it from mains you can still self charge and choose where to use the electric power - i.e. siwtch it to charge on the motorway then switch to EV mode around town for a nicer drive and more overall efficiency. Most people would not bother doing that though.