Well, it seems that we're all over the map (the range seems to be about +/- 2.5mpg), so I'll just wait for a few more tanks to get a better feel for how much mine is off.

In the end, it doesn't really matter, as long as the instruments are consistent; they're still useful for judging my mileage relative to other trips in the same car, and that's the most important use I have for them.