Although the cooler temperature causing issues was already mentioned, I think it is important to remind people that a conventional gasoline powered car uses "waste heat" to warm the cabin space, therefor it is "free" energy that would otherwise be removed in the radiator. That is a large reason why your small gasoline and diesel cars don't see such a dramatic decrease in MPG in the winter, whereas EV's PHEV's and hybrids that use an electric resistance heater are going to suffer quite dramatically. It is ironic that the inefficiency of the internal combustion engine leads to a lot of extra heat, which we have been harnessing to warm our cars interior. The same issue will arise with the ICE if we ever produce an ultra efficient gasoline engine that doesn't produce heat. You will need an electric heater, or some other way of heating the interior of the car. All effeciency technologies will suffer from this same phenomenon, air cars, hydrogen cars, electric cars, flywheel cars....