This page will answer all of your questions, and will explain, in simple language, the entire problem of auto gas mileage. 

This site will cover the two factors that cars must overcome, that is, rolling and air resistance, and it will explain the third factor, how the automakers design cars and their powertrains to overcome these elements. 

You will find examples and theories based on sound engineering, experience and logic, and will also teach you a little about combustion.  This will put you head and shoulders over almost anyone you know about this subject.  Learn as much as you want, or just read the key points in BOLD.  Its all here.

Now, here is the takeaway, the main learning point of this entire site:  Your car engine burns fuel to generate power.  Power is needed to roll the car on the ground and push it through the air.  When the engine is efficient and the gears keep the engine in its optimal operating range, the car will get good mileage.  If its too heavy or awkwardly shaped, or both, the mileage will be worse because the engine will have to fight those elements harder.

Again, the two things your car fights are:  1) the road and 2) the air.  These are the only two phenomena that your car comes in contact with, namely the air around it and the road that the tires touch.  Thats it.  Think about it, though; what else does your car come into contact with?   It contacts the air and the ground.

To fight the air resistance and rolling resistance, your car must have an engine and gears, the things that can fight the resistance, accelerate the vehicle, and then keep it moving.  For a given vehicle of a specific size and weight, it is the extent that an engine and drivetrain are efficient and well-matched that determine the actual mileage.

Well get to aerodynamic drag later, which will be fun because you have all seen commercial advertisements that talk about drag coefficient ratings and wind tunnel tests of cars, and the like.  Following that, I will explain engine designs.  After all, engines must overcome the rolling and air resistance.  Then, just prior to the overall summary, I will present examples of mileage 15 years ago and today, and will even post reader experiences and opinions.  The overall summary will tie all of this together.  In the end youll know a great deal about both types of drag and car engines.  Numerous examples will solidify your knowledge.