got the idea from Brucepick. anyway, in theory, pumping gas slowly prevents vapors from escaping resulting in more gas retained.
the first time i did this, my FE went down significantly. IF more gas is retained, this makes sense via more gal to divide into miles. next fillup would balance the #s right?
this IS what happened to me. got ~30mpg before the test w/ VERY favorable conditions. then, pumped slowly to~25. next fillup, again slowly, got 29+. both were less favorable, but VERY similar miles, w/ the only variation being my lower grill block. it came off during both "trips," the latter being repaired(replaced)permanently. oh yes, and one extra day to work(mostly hwy) on the 2nd. that can't be worth 4mpg tho.
It might be that pumping slower means the fuel foams up less and you can get more in the tank before it clicks off. Getting more gas in the tank would make it seem like you got worse mileage. Can't say for certain that is what is happening though as one tank isn't enough to go on.
Usually pumping slowly will get you more fuel from it leaking around the metering system in the pump - the amount of fuel vapors that come out of the tank while filling is next to nothing. Today when I got gas I usually pump slow for the last gallon into the tank because it foams up so much and when I did the pump did shut off then the gas over flowed - I still put in another $2.00 work of gas after it overflowed - sure wish there was a better way to get the gas in with it not spilling and still fill the tank up.