[instarx]

My Volvo's computer consistently indicates worse average mileage per tank than I've really gotten. it's usually off by about -2 mpg. The speedometer and odometer readings are exactly right as measured by my GPS, so it isn't some strange tire diameter issue.

I would guess that OEM computers use a standard factory calibration value that can be a bit +/- for any particular car (but 5.5 mpg does seem like a lot). An after-market guage could be better or worse, depending on how well you calibrate it.