No matter what happens, the best you can expect is to coast to a stop. The car is not designed to drive on electric power alone and I would be surprised if the engineers even allowed the assist to keep working without fuel, for the sake of the battery. Heck, even a Prius really can't be expected to go more than a half mile or so and at the risk of over-discharging the pack. My recommendation: Don't run out of gas no matter what you drive.