It is really easy to get an extra 1/2 gal or more into your tank if the car is not sitting level at the pump. Or it will click off when you're 1/2 gal or more short of a "full tank". That's why it's risky to talk about your mileage on the basis of only one tank. I have calculated tank averages that are at least 5 mpg higher or lower than indicated by Trip B (which I reset when filling the tank), but my long term average (40000+ mi) is very consistent - Trip B reads 3.2 mpg higher than my calculated values. I believe the trip computer is pretty reliable, but it just may not have the absolute accuracy which we would all like, and I'm pretty sure it's accuracy depends on Civic model ie '03 vs '05.

Sorry this isn't on topic, but it's been on my mind for quite a while.

Dave