This is one big reason I like EV drive, flex fuels. First EV is very efficient at high AND low outputs. (85-98%) The problem is that generating and storage is not. (7% combined) Still if you get to charge at home at night (and not too many people are doing it), that can make up for a lot.I wonder if a fuel cell - like one from Intelligent Energy - would make it more practical?
However, there isn't much reason you can't have different forms of generation. Ideally if an EV platform was modular in the right way, you could swap out your battery pack for a fuel cell, even different types of fuel cell, or to a generator for CNG, Propane, Diesel, petrol, etc. Or some mix that suits your needs. A half-sized battery pack and a small top-off fuel cell, for example. All this while keeping the same basic drive train.
Anyway, that is possible in the kind of spaces I put in my Atlantric. It was just a by-product of making the body a full streamliner shape. I'm not sure how I'll continue on the project, but flexible fuel is a possible goal.
One of the things I did back in the early 80's was multi-fuel carburation. (SAE paper 1984) It was a system added to the top of the carburetor. So you could switch-in-route from petrol to gaseous fuels, in this case a mix of propane and CNG in the same tank. It would run on cheaper CNG until that ran out then the liquid propane would start to boil, and you'd run on that. A feed-back ZOx sensor would signal to adjust for the differences in the fuels.