Going by the H3's analog gauge and the odometer vs. how much gas the attendant squeezed into your tank (and filler neck) is not an accurate way to calculate fuel ecomomy and leaves a lot of margin of error.
How do you know your tank was absolutely full prior to the trip? Because the pump shut off? How can you verify that when you refilled your tank was top-offed? Unless you weigh the volume of the gas in the tank theres no possible method to insure that you are returning to that same volume. Plus you only drove the hummer on your one way trip with the additive. Theres no data about the same trip without for comparison. Depending on driving conditions you might have got the stated MPG regardless.
You really need something like a Scanguage II that hooks to the OBD to read acccurate MPG directly from the CPU.
I have used one, and I can tell you my H3 can get 97.2 MPG!...with a tailwind, coasting at 65 miles per hour down a 6-degree decline. I'm apparently also heavy with the foot off the line at redlights resulting in a measly 4.2 MPG. Good thing I'm not constantly accelerating. I guess thats why they use averages...
