I usually divided it by the capacity of the tank, now how much I put in...which should be about the same tho right? which is more accurate, cause says you put in so many gallons, there was still some in there already...

 

No.. lets say you filll up at half tank. Would you divide 150 miles by the full capacity of the tank? Then you would be getting like 11 mpg... It is an extreme example.

You divide how many miles you drove by how much gas you put in. Period.

 

Ok the flat out way to figure out how much mpg you get.
Amount of gas put in at fill up / Miles driven tell fill up. a ex:

I go to the gas station & fill up my tank, it takes 10 Gallons to fill up. Now from the last time I got gas tell now I drove 220 miles. So I would do 220/10=22mpg. Again this is a example. For a general thing that might help you more I will copy over my gas milage from my current car. The milage wont be like what you get but it might help you understand the math.

Gas Milage
118,109 - 117,755 @ 12.304 = 28.77 mpg
119,338 - 119,090 @ 08.606 = 28.81 mpg
119,609 - 119,338 @ 08.158 = 33.21 mpg
119,857 - 119,609 @ 08.560 = 28.97 mpg
120,043 - 119,857 @ 06.488 = 28.66 mpg
120,287 - 120,043 @ 08.608 = 28.34 mpg
120,515 - 120,287 @ 07.529 = 30.28 mpg