"Ethan - don't forget topography as well (think San Francisco vs Topeka). But you've listed some good variables that aren't always obvious to some."
As I originally stated, "there are many factors that affect the mileage of a vehicle, 'including but not limited to'...". There are of course more factors than what I mentioned, such as topology, type of fuel used (formulations varies by region and throughout the year), and maintenance (air filter, oil, etc.). The importance of what I was trying to say is that people need to consider the many factors that influence gas mileage when considering the numbers and not compare directly with other people's numbers when all factors are certainly not equal.
Your observation about topology deserves further attention. Consider that work is defined as force times distance, and that when a car does work it consumes gas. The more work is does, the more gas it consumes. When you're driving on a flat surface, the amount of work your car does is basically equal to the sum of forces (drag, frictional losses in the drive train, tire resistance) required to propel the car at a constant speed multiplied by the distance that the car has traveled. When driving on irregular topology, the car must change elevation too. The amount of work done when climbing elevation is equal to the weight of the car multiplied by the change in elevation. Of course, roads don't go straight up or down, so the real work done by a car while traveling on hilly roads is equal to the work required to move it forward plus the work required to gain elevation. It should be obvious then that even a small change in elevation requires significantly more work, and thus consumes more gas, than driving on a flat road. Also, you don't get back the work done while climbing a hill when you go back down, though gravity does help reduce the amount of effort required to keep the car moving forward at a constant speed. In conclusion, the heavier the car or the hillier the terrain, the more gas a car is going to consume, regardless of the type of car. Our cars are relatively heavy, and so anyone who drives in a hilly area needs to understand that they will get significantly worse gas mileage than someone else who doesn't drive many hills, all other variables being equal.