Our cars have 'reserves'...I don't know wtf that means, but I assume it's about 1.5 gallons of fuel that your gas gauge doesn't register. The most I've ever put in my car is a little over 16 gallons. I ran out of gas once but that was because my gas needle stuck on my indiglo gauge face h:

Our gas gauges are screwy...I've spent many a mile studying it. They tend to stick around the full, half empty and empty marks for a loong time.