I've got to second Zorak, here... Please don't do it. Not to your VX, at least.
I think the best way to get more mileage out of your gas is to use real gas. That is, try to find gas without extenders in it. E85 and other ethanol blends have less energy content than pure gasoline. However, now days, I have no idea how to FIND real gasoline. All of the pumps in Denver say "ETHANOL: This gasoline is oxygenated and will reduce...blah blah blah".
Even more interesting to me is the sticker on the pumps at Costco. They say they sell by volume, uncompensated for temperature, and a volume does not guarantee a specific energy content. Seems to me you should do you shopping at Costco when it's cold...