The Volt: 90% EV, 10% ICE for all practical purposes
Technically speaking, Chevy Volt is a PHEV as it uses both an electric motor and a gas AKA Internal Combustion Engine (ICE).
It also, for the most part, acts like a series hybrid as it uses the ICE to generate electricity for the electric motor and not to drive the wheels directly, just like the BMW i3 REX. However, it is not strictly/technically a series hybrid as the ICE connects directly to the drivetrain in extreme circumstances e.g. when the battery is almost dead and the car is cruising at a steady higher speed to improve cruising efficiency. This means, for its estimated 40 mile range (spot on, as research confirms most e.g. 78% of Americans drive no more than 40 miles a day on average), the Volt will not only be powered by its electric motor, but the ICE motor/generator will be completely off.
Also, unless the owner is completely clueless and decided to spend a lot of money over a standard Chevy Malibu Hybrid (which is based on Volt) for nothing, that owner will regularly plug the car in and use it almost exclusively as an EV. At the same time, most owners of limited range cars like a purely electric Leaf also have an ICE-powered car for long trips, so it's not like such EV owners are practically using 100% electric power either.
There is no shortage of blogs and owner accounts that confirm cars like the Volt essentially match EV's in terms of advantages.
One such blog is is Voltowner. Such blogs confirm over 90% of the miles driven by such owner in the Volt were all electric and oil changes and other engine maintenance upkeep occurred some (1-90%)/1 = 1/10th as often as with an ICE. So unless you consider something like an oil change every 30000 miles or so typical hybrid ICE engine maintenance, you might want to consider a vehicle like the Volt as possessing virtually the same zero-maintenance characteristics as an EV.
Far as mileage, rates above 200 mpg are fairly common and Fueleconomy.gov rates the Volt at 149.7 mpg off gas. How? Because, again, the majority of energy "burned" is electric.
A true test of how efficient Chevrolet Volt is would likely involve tracking all the miles done by several Volt owners vs. several entry-level pure EV owners, such as Nissan Leaf owners, between all the cars they own and calculating how much gas, in total, was used to power them per mile driven. I am betting the gas burned per mile would be approximately the same in total and the split between EV and ICE-fueled distance would be about the same.
For these reasons, among others, I believe the Volt acts as an entry-level EV and, with all but the most careless owners, will match the environmental savings of a pure EV. Thus, I also believe it, for all practical purposes/usage, should be considered a pure EV.
However, to note, I believe the Volt should have its own subcategory relative to pure EVs, which is the "Gas-Extended EV". The reason being that, in a rare pinch or low battery situation, the Volt can be refueled/powered by gas, unlike a pure/battery-only EV.