I guess I tend to exaggerate. Yes, that's exactly what happens, I fill it to the neck and the gas recedes. I was in a hurry this morning, and saw it took 3.5 gallons. I didn't try to top it off this time. Assuming I still had 1 gallon left in the tank and it holds about 5 gallons. I assume I would have been able to top it off another half a gallon. So, I guess this is normal. Thanks for the info.