For you to know this you have to know how much your car can hold which is usually pretty easy to figure out see how many gallons you get when you are empty. Then you can figure out your miles to gallon by tracking your miles on your speedometer to when you next need to fuel up. Miles drive divided by gallons in gas

MPG times tank size

there is no one answer for this

that can vary greatly depending what type of car you have

some gas tanks are bigger some have better mpg

there really is no such thing as an "average car"

average tank 20 gal, avg mpg of your car???? prolly somewhere in the ballpark of 20...do the math ;)

What are you driving? A moped, muscle car or 44ton truck?