While you are right that it would not be deniable that the FE has gone up, it would, however, be impossible to convince the local peanut gallery (that includes me, btw) that the FE improvement was only attributable to one specific change. Especially when you spread out the measurements over a long period of time... tire pressure, tire wear, weather (winter vs summer temps), fuel quality, engine condition, travel routes, etc, etc can all have a significant impact on your FE over time. You can look at my fuel log (and I'm sure many of the ecomodder member's logs) and you can easily pick a range where 15 fills (each fill is after about 900 miles for me) is lower (or higher) than the next 15 fills, despite no specific mods being done.
I think I'm just restating what most have said. Would love to see the results for this, but if you are planning to do it by doing 30 fills before the mod and then 30 fills after the mod and assuming that's good, then I'd say don't even bother. Just do the mod, report back the results, and everyone will take it with a grain of salt or totally ignore it (like me). If you want to provide some believable results, then it will cost you some time & effort & $$.
I will freely admit that I had grand plans to test several mods on my cars with ABA methods, but never was able to find the time to go through that kind of effort... My hat's off to those folks that have done a reasonably thorough job of testing different mods... I do use the results when deciding what to do next on my cars. I've done several mods, some cheap, some expensive, and I do believe the mods have helped to improve my FE, but I wouldn't tell you that
x mod improved my FE by
y percent... I just dunno
