After 2,456 miles my display (trip "B") shows 60.1 mpg and the database here (based on actual fillups and trip mileage) shows 58.4 mpg. This makes the display in my MT HCH 1.7 mpg or 2.9 percent off which makes mine very close to Lars-ss'es (OMG that is a lot of "s"es) as his is 3.0 percent off (1.43 / 47 * 100).

Actual fill data is right on the money in the long run but is useless for tank to tank data because of differences in fill level. The gauge display is not useful for most of us because they tend to be optomistic. Neither way is perfect. The best way is to use corrected display figures but that is a bit complicated and impossible in the short term.