The cost to fill your tank.

People. Whether it takes you $30 or $50 to fill your gas tank, it has nothing to do with your gas mileage. It simply tells you how big your gas tanks it. Gas mileage is the number of miles traveled divided by the number of gallons of gas used. The size of your gas tank may be arbitrarily related to gas mileage in that the manufacturer doesn't want you to have to fill up too often so they will put in a bigger tank if you get bad mileage, but it is not directly or scientifically related to mileage. So stop bragging that it only takes $30 to fill your tank. That means nothing.