Okay, I finally filled up my tank after spending the last one without the radio, CD player, A/C and heater... no gunning the engine, altered work route to take expressway instead of freeway (note: I did have to turn on the fan for about 15 min). So, my efforts with hypermiling produced the following numbers:

Manual calculation (total miles / gallons filled) = 26.17 mpg
Energy Monitor calculation = 27.5 mpg

=> Energy Monitor 5.0% higher than the "actual" mpg

So, how does this discrepancy compare to the tank prior to this where I had the CD player playing all the time, drove on the freeway for work commute, gunned it here and there? The numbers there are:

Manual calculation = 24.8 mpg
Energy Monitor calculation = 26.1 mpg

=> Energy Monitor 5.2% higher than the "actual" mpg

Namely, the discrepancies to within expected uncertainties are identical. I measured similar discrepancies for other tanks as well.

In other words, you should expect your Energy Monitor measurements of your MPG to be about 5% higher than the actual values.

(Would be interesting to know what percentage of the numbers stored in the mileage database are based off the EM versus manual calcs).

I do have one measurement that indicates that this discrepancy can be additionally about 2% higher when you've used the A/C and/or have a trip with lots of elevation changes from climbing/descending hilly streets. I'll keep track of this during the summer months to see if this is real.

Anyways, I'm going back to enjoying my HiHy; cranking up my CD-player, my butt-warmer and occasionally outgunning those obnoxious Suburbans. Hypermiling is fun... but only up to a point. Always holding back 268 hp is just plain cruel. As long as I know the EM is always about 5% higher, I can at least know where I really stand (i.e. keep the EM above 26.25 mpg (=> 25 mpg actual)...