The gauge is 'designed' to read empty - 0% - while still offering a reserve tank capacity of somewhere between 1 pint and 3 gallons. It's a safety feature to ensure the driver is alert at all times.
Truthfully, though, the manual does say there is a reserve after 0% and that's fine on mine. I've done 50 miles on 0% and still couldn't get more than 6 gallons in. My gauge reads different figures based on slopes and acceleration / deceleration but apart from that is pretty consistent.
I did find when I tried to brim fill the tank that petrol spilt a bit and the car misfired for a few miles. Again, the manual seems to say don't brim it completely. Seems it might cause a few problems - airlocks?
