But as we all know driver style (heavy foot/slow acceleration) and location (hwy/city) have more of an impact on mileage than almost anything else, so unless there are huge differences I still say it is possible that most positive results are placebo effect (aka high hopes = slow acceleration = better mpg).
its true.. i was in a very relaxed mood today.. my trip home i averaged 18.3mpg according to the scangauge (which has been dead on accurate to the .1 gallon over a full tank for the last 6 months) and on other days i am as low as 12.5mpg on that same trip