Quote from Sandybestdog:
... With the Prius plug in, yes if you run solely on battery, it will be comparable to the Volt. I think there can be some debate about 40 miles being the correct range for the Volt but I think there is no debate that the the Prius's 12 mile range will constantly leave you needing to use gas, which must be factored into total cost. Also, even in electric mode, at 62 MPG, the Prius must use gas. There has been some debate but officially, while in extended range (gas) mode, the Volt's engine generator simply recharges the battery and then the battery powers the car. So there will be times when in gas mode that the engine is not running because there is a enough of a buffer in the battery to power the car at that moment.