Since GM produced a car that only gets an EPA rating of 35MPG and a fairly sad(IMHO) 360wh/mile, I think there is room to improve. The Volt produces worse efficiency than almost every conversion I know with a car that they tout as their most aerodynamic car and it weighs in at a hefty 3781 pounds while they are at it. I do give them kudos on their crash safety rating though as it is one of their safest cars so that explains at least a good chunk of weight.
The Prius plug-in is a bit of a joke, if you don't know what I'm talking about, drive one and you'll get a good idea, getting it to stay in EV mode can be a challenge and once it fires up the gas engine it stays on until the cat fires when you didn't want it to start in the first place.
These are the only two major plug-in hybrids on the road that are available from a manufacturer (excluding the Toyota third party DIY kits) allow for some kind of pure EV mode.
The idea of a plug-in hybrid isn't a bad idea if an efficient car can be produced in the end, which requires a lightweight car with good aerodynamics and LRR tires that are legitimately LRR. It is a challenge though, GM seemed to really put their focus on battery longevity with their pack usage limits but then again I can understand if someone drives 50 miles a day and cycles the pack to GMs set limits daily why it is done.
If I did anything plug-in I would find a very efficient small gas engine appropriately sized for the average highway load and geared for the max speed I'd drive on a highway trip, I would use the electric power to get up to speed, start and engage the engine and use it on the trip while at highway speed and use the electric whenever the demands exceed what the gas engine can do. I'm talking interstate driving here. In my case the engine wouldn't climb a mountain, that would be out of its scope but I'd use a 1st Gen Honda Insight which is what I'm converting anyway if I were to do such a thing. Instead I'm not doing that, I'm going pure EV and aiming for 100+ mile range on a ~20kwh pack. There won't be room for a gas tank in the car and probably no space under the hood for augmented power once I'm done and I refuse to give up my spare tire. With less range packed in the car I could probably fit a gas tank but the whole idea won't be touched by me.
Good luck, sounds like an excellent opportunity to learn and demonstrate what you can do.
