So I filled up tonight. I have a ScanGauge II in my truck, and it's been pretty good at estimating the amount of fuel needed to fill my truck. I thought I went through quite a bit in the last couple weeks, so I was interested in the fill.
My ScanGauge estimated 45 litres to fill my tank. In actuality, it took 66. 21 litres difference! Interestingly enough, 21 litres would be a 5 gallon gas can. I have a locking gas cap on order, to curb the continuation of this.
Just be warned. If you live near Centre St. or Edmonton Tr. in the North, lock your gas door, or get a locking cap. They are out there.