For me, it's not knowing what the 'formula' is. It's more of what people do to try to keep the calculation as accurate as possible. Refer to the OP for further explanation. Like when do you fuelup, the moment it hits E or at a certain mile range left? Is everything 'ruined' if you fillup at 1/8 tank as opposed to E?

I guess what I am saying is, how do you accurately gauge how much gas you used?

here is an example. On monday, you drove 200 miles and used 10 gallons of gas. 200/10=20mpg.

on tuesday, you drove 200 miles and used 11 gallons. 200/11=18.18.18.18.18 (I did not plan that lol

but on wednesday, you drove 200 miles and used 9.5 gallons. 200/9.5=21.0526

So, the amount of gas you put in the tank on every fillup does not matter as much as you might think. it all works out in the end.

If you only checked your MPG's on one tank for the life of the vehicle, it could come up with a wildly inaccurate number, but in the long term, it doesn't matter. Weather, road conditions and even your attitude at the the time will affect you MPG more than the little difference in the little bit of gas you put in the tank.

and this, coming from an MPG nut...

ok a little more clarification.

total of those 3 trips, 600 miles. total gallons 30.5

600/30.5=19.6mpg