I attribute the fuel economy gains to the open exhaust, combined with ram air intake, and the changes my friend made to the computer including advancing the ignition timing. I use the Bosch platinum 4 plugs, they say you can get better mileage in their ads, I can feel a difference using those plugs in most vehicles, smoother idle and less hesitation, quicker starts, but as for their effect on economy, not really sure since none of my vehicles are stock, and most have been modified for a combination of power and fuel economy.
I do have an old 91 mazda 323 that I use for running errands in town, fishing, and just plain hauling dirty, ****** stuff around like topsoil and mortar mix. It uses similar mods to the Ram truck, (+4 plugs, ram air, no muffler or cat, no egr) and it gets 42 on the highway, and 36 city. I love rubbing my friend's nose in that... he just bought a brand new hybrid that gets 42 mpg... I always say, "and what did you pay for that? My $400 POS gets 42 mpg and it's paid for!"
It's become our running joke, lol.
Dave