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if your car has a decent evap system (like the old honda insight) you get elevated MPG figures for a while after filling due to the vapor in the carbon filter. If you don't drive far your tank stays near full pushing on the evap.
I am "uncertain" if the effect matters much as you are just burning gasoline that evaporated in your tank but if it improved how the vapors burnt by being so much rich, I have no idea.
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