Whilst you might not have been able to fill it up as much as you estimated, you car is probably doing more MPG than you first thought. I find my fuel gauge can drop suddenly within a few miles, then as soon as I find a constant speed it rises again. At best the fuel gauge is an indication of some petrol in the tank, often beyond that hopeful note of optimism , I tend to rely more on the miles travelled since last fill up and my conservation MPG estimate as my more reliable gauge of remaining fuel, as I don't need the onset of remaining range anxiety.
