One of the least accurate instruments in a car panel (an analogic panel, at least) is the fuel indicator. Not only it is pretty small, with little marks on it, but it's value depends heavily on the position of the car. If you are on a inclined place, for example, it you vary for more than 10% (mine does, I swear)

So, how can you know how much is your car consumming?

Just fill your car tank, to the top. Ok, now write down the marks on your odometer. Or press that nice button that clears the second odometer (you know, that one that is usually at the bottom of the car panel).

Now go around for a ride. Go visit your mother, father, friends. Or just go to work. For maximum accuracy, make the same path that you do daily, at the same time.

Now go back to a gas station and refill your tank. Write down how many gallons you spent on that little ride. Now see how many miles you walked. Now just divide one by the other (the miles by the gallons, unless you drive a F1)

That's it... now you have your fuel consumption, easy and precise