All my figures are by gas pump and miles driven via the Odometer - the hand method. This is a proven method - my Dad was doing this long before the scan gauge or other tools appeared.

If you approach it from the standpoint that:
1> your odometer is accurate (why would it not be) and
2> the pump is accurate (weights and measures officials routinely check gasoline pumps for accuracy every year)
- then even if the pump shuts off early or late - in the long run it all evens out.

In addition to posting my results on this web site - I keep an Excel spreadsheet. In the spreadsheet I have a column that takes my last three fill ups and computes what I call my "rolling mpg". The peaks and valleys of individual fill ups are not as pronounced. With this calculation - it basically eliminates fill up variations from different pumps and locations. It also allows for a smoother trend line then individual fillups.