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I don't think we can 'learn' much from taking mpg readings from one tank of fuel... sure, we might get bragging rights that justify something to somebody (may only ourselves) but we simply can't do accurate enough filling in one tankful to defiatively learn weather or not that latest driving technique or do-dad made 1mpg difference or not... the sample size is too small, the equipment not accurate enough, and the environment is not controlled enough... that's the bad news.
The good news is this, so long as all the fuel from the pump goes into the tank, (not spilled on the ground) and the pump is measuring accurately (controlled calibrations performed (monthly?) by the State Board of Weights and Measures) averaging takes care of the uncertainty of 'how much fuel did I just burn?'. Through averaging you will always get an answer to the question "what's my mpg?" your real fuel usage, which is what you care about anyway.
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