Spending money to increase fuel efficinancy its like fighting to peace.
How much are you getting per tank and do you know your consumption per 100km?
Before I wrote my VS off, I got from Hornsby Nth Syd to Coffsharbour and it was aprox 540km, and I had used only 41 litres. Pure, highway cycle.
Usually with the VS's, are the O2 censors, crank angle sensor and the fuel pump affect their consumption.
Your trip is well over now, but tell us how it went?