I have a 2000 Grand Prix GT that is supposed to have an 18 gallon gas tank. I've been running my own "experiment" lately because a friend thinks my fuel level sending unit is bad. If I let it get down to where the low fuel light comes on and fill it to capacity, it only let's me put on 12 gallons. I can't think 1/3 of my gas tank is low fuel, so where is the missing 6 gallons? Is it my sending unit on the fritz? I feel like I fill up more often than needed. Thanks!