Well guess Mike C. would find me a real idiot. As I claim to get 1.5MPG improvement with the Stanadyne. Either that or I am simply ignorant of all the possible parameters effecting fuel mileage.
All I know is that I have repeatability. That is I repeatedly get that increase when using the additive and repeatedly loose the increase when I don't use the additive. I do have 94300 miles on my F550, so I am probably well beyond break-in. I am aware of the effect of speed and wind, which is why I have checked the MPG when holding my speed to 60, 65 and 70 MPH. This across the same terrain within a time frame of two weeks in one recent case and three weeks during another recent test. Of course I was not equipped to measure barometric pressure, air temperature under the hood nor most certainly air density. But still with additive I see an increase of 1.5MPG over not using the additive. Though the mileage improvement was not my reason for beginning to use nor continuing to use the additive. Is is most certainly a side benefit.
As to how the additive improves mileage, all I know is that it says it increases the 'Cetane' of fuel. I have no way to prove or disprove this claim. As for the major trucking companies not using such an additive there is for one the fact that their mileage is low enough to begin with that any improvement might prove to be negligible. I am going from 13 to 14.5 which while roughly 10% and worthwhile even at 3.5 - 4.5 MPG, don't know as it would have the same effective percentage increase. The major trucking companies are of course running the larger Cummings, cat, IH and Detroit diesel, I am running a 7.3L V8 Powerstroke and have no idea what if any difference that might contribute to the numerous other factors even assuming for the moment the additive does in fact increase the cetane. Would a small increase in cetane have the same impact on in line six versus a V8, etc? Don't know. Don't care. Know the results I consistently get. Know that when I stop using the additive the MPG goes down. Start using the additive again and MPG goes up. I hand calculate for each tank. Kept a spreadsheet for sometime, unfortunately lost the spreadsheet when changing to a new computer. So now have begun a new one but only have three fill ups currently logged.
Now the really funny thing, is I am usually on the side of Mike C. in these discussions and for the most part still am. I believe they are mostly if not all marketing hype and of little to no real value. Like I said I started to use the Stanadyne because it was recommended by a well respected and experienced diesel mechanic (actually owned the shop). Taking his recommendation and the fact it is the only additive I know of both recommended and sold by Ford.
As an aside one of the reasons I was checking the mileage at different speeds to reestablish the MPG difference at various speeds, primarily 65 versus 70MPH. In my case the F550 with RV hauler bed but not loaded or towing with two passengers was 2MPG. Ok, actually 1.8 to 2.1.