Just a clarification, I'm *not* basing this in any way on the fuel gauge (directly). I'm going solely by the "average fuel economy" reading on the "message center" (what the FEH has to display a teeny bit of information to us cheapies who did not go for the fancy "Nav/Audio" upgrade).

I know that over an entire tank, the average FE gauge reads a little higher than what I get by calculation (e.g., it will say "33.4" for the tank, but calculation will show that I actually got 32.5, or so - the difference tends to be a bit less than 1 mpg). What I don't yet know is whether the degree of that error is greater at the beginning of the tank, such that the high readings I get for the first 100-200 miles are simply more "wrong". By doing a few 150 mile top-offs, and calculating, I can determine if that's so.

I had considered that the average FE gauge must base the number that *it* displays on the number of miles driven (obviously calculated from odometer reading or same source) and the amount of gas used - and that the latter figure might come from the same data source as does the readout on the fuel gauge, so any inaccuracy in the latter (which is obviously non-linear) would affect the former. HOWEVER, the fuel gauge in the FEH is non-linear in the opposite direction from most - it goes down FASTER at the top; several other FEH owners here have confirmed that. So that kind of error would tend to create the opposite result from what I've observed - the FE would start bad, and get better.
I'll report on my conclusions after I've determined whether the high readings actually correlate to higher FE as calculated - it should take me a few weeks to get enough data. I'll also try to keep better track (i.e., write down, not rely on memory) of where and how I'm driving over each "mini-tank".