My daughter and I have slept in both a small car and a minivan, and being petite, we fit comfortably.
The mpg of a vehicle also depends on what you are carrying in the the vehicle. If we travel light, this will help. We won't have to carry tent equipment, and the heaviest item would be a small cooler for cold drinks, etc. We will just have to watch that we don't carry too many heavy cans, etc, so we don't hurt our gas mileage.
Determined Duck