Does it matter how many people are in a car as to how much gas the car uses?

this is pretty easy to figure out, the answer is yes, if you had to haul around ten pounds as opposed to hauling around 70 or 80 pounds you would have to use more energy as well, same principle works for an engine

Yes! Take your wife, the tank will be empty by the end of just one hour

Yes, but not, like twice as much. A little more.

as long as your going downhill