Fuel problem

So I screwed my fuel gauge up so it does not give me any type of accurate reading. So I filled up my tank, reset the odometer, and drove until i ran out of gas, pretty much died right at 350km's. So i have been basing my fuel consumption off of my distance travelled as well as my economy gauge. I recently Changed all the fluids,plugs, filters and all that jazz as the car is new to me. I am finding that I am now getting way more than 350km's per tank, I have been going off instinct of when to put gas in.The other day i put 20 bucks in, drove out to tabour lake like 4 times and drove around a bunch more over the next few days(about 180kms). Seeing as how my tank wasn't full when i put the 20 bucks in I figured I should fill up. I go to fill up and I only get 14 bucks of gas in until its full, and I mean full. I am now finding I am getting around 450kms per tank. I did recently move so I am not hitting the highway as much, but damn I still drive around. I am basically chalking the change in fuel consumption up to a major change in my driving as per speed and what areas of town i normally drive in (no hills). Anyone else have any other input that could explain this other than I'm a retard who doesn't know how much fuel was ever actually in the car? This is for the beemer btw.