I have a 2000 Grand Prix SE and car specs say that it has a 68 liter gas tank. I converted that into gallons, and its about 17.8 gallons. when i refueled, the gas pump only said it put in 12 gallons when the gauge said it was running on fumes. How do i fix this problem?

Make sure you fuel tank float isn't becoming saturated with gasoline. If the gauge reads full when it is indeed full, you probably don't have a short in the wire that goes to your fuel float sending unit. If that wire is bare and shorts out it will say you tank is empty since it works on resistance open or unplugged the gauge would read full, closed is or going to ground the gauge will read empty. I would think the float might be bad or the float arm got stuck down when you turned a sharp corner. I had one of these cars but I don't remember exactly where the fuel level sending unit is located and if you need to remove the fuel tank to gain access to the sending unit. I hope this helps you

Your tank's supposed to have some head space so fuel doesn't slosh out, but that's usually 3 gallons...the fuel pump's not so big even though it's inside too. That makes it 3 gallons (20%) off...bad enough!
Ideally, you'd get a chance to see an endoscope (from a garage or school) thrown down your tank to see how much is really left; even failing that, there are several inexpensive replacement tank level sensors (and displays connected with twisted pair) out there. Then you wouldn't need to follow replacement/recalibration procedures for your existing sensor and its electronics.
According to the below, you should get some gas tank cleaner and see if things clear up (esp. if you didn't already buy a tank of premium gas.) After all, a replacement sensor is $50 or so...and you'd have to sump out the tank to swap that (see last item.)
http://www.carcomplaints.com/Pontiac/Grand_Am/2000/investigations.shtml http://wiki.answers.com/Q/2001_grand_prix_GT_fuel_gauge_not_reading http://wiki.answers.com/Q/How_do_you_fix_a_gas_gauge_on_a_Pontiac_Grand_Am