I added an AEM short ram air intake system to my 1993 honda accord, as some of you who participated in the "any modifications to increase fuel economy?" thread may recall.
Last time I averaged out to 35.625 miles per gallon. 285 miles on 8 gallons.
Well today I filled up at the same gas station, using the same pump to eliminate variables (I rely on when the gas pump stops flowing to give me a fuel consumption #). 103.5 miles at 2.6 gallons.
That's 39.8 miles per gallon! My normal commute did not change, it's still 3 miles of highway, and 7.5 of normal secondary highway with 5 traffic lights in all.
It's possible that the previous air filter was pretty dirty though, and so replacing it with a stock probably would have increased fuel economy as well, though I doubt as much as this one.