I'll second the "are you sure you're filling it up the same way each time" point.
I fill to the brim at the same pump, same direction each time, so I know my calcs are as close as possible.
And also, yes, you can get accurate fuel economy instrumentation for less $ than a
ScanGauge: there's the somewhat DIY route of the
MPGuino; also, look up the PLX Kiwi- they have a less expensive than
ScanGauge model.
Highly recommend you get
some fuel economy instrumentation. It makes the game a LOT more fun, and it'll pay for itself many times over, seriously.