Why do I have to fill the tank all the way up every time I buy fuel?

The only way to measure fuel consumption without complicated gauges and meters is to measure it by difference from full. So say I have a 10 gallon tank and I drove 180 miles and it's nearly on empty. I fill it up (9 gallons worth) and find out I got 20mpg (180/9 = 20). Now lets say I only drive 60 miles and I get gas again. Lets say to fill it up, I only need to buy 3 gallons of gas. That also gives me 20mpg (60/3 = 20). This all works fine no matter how much you drive and how much fuel you buy, as long as you fill it back up when you go to a gas station. That makes sure we all start from a known quantity of maximum. The problem is when you fill up, drive 180 miles, then only buy 2 gallons of fuel. Computing correct MPG from that is impossible, because it would appear as if you drove 180 miles on 2 gallons of fuel (resulting in 90mpg). If you filled up soon after, you would likely have driven only a few dozen miles and needed say, 9 gallons to fill up. That follow-up calculation would also be way off, coming up around 2mpg instead of 20mpg. So, to correctly track your mileage on this site, always fill your tank all the way up. It doesn't matter how much fuel you used as long as you track the miles driven (via odometer or tripometer readings) and the amount of fuel taken to get back to full, it will work out.