[puddleglum]

First, (unrelated to your SG) if you have an evaporative emissions system on your car, be careful about filling right to the top of the filler neck. You could flood the system and do some expensive damage.
As for SG, what is it saying for your tank avg. mpg? Does your SG odo match up with your dash odo? It sounds like a calibration issue to me. It took me quite a while to get mine set and I still have to adjust it from time to time cause it seems change.