Why can you only ever put in 14 gals of gas into your 1999 Pontiac Grand Prix GT coupe when the manual says it holds 18 gal?

Answer 1

1) you have never driven it until the tank is completely empty. 2) as it gets close to full gas can't leave the filler tube at the same rate causing the overflow valve in the pump to shutoff. Try filling at a slower rate at the end. 3) If it's a metal tank it migh have gotten buckled one way or another so that it can't hold the full volume any more. 4) the documentation is wrong 5) or your car isn't like all of the other cars. I should have included in the question that the reason for filling the tank is that the fuel gage reads empty, the audible signal has beeped & the word "Low" is permanently displayed on the dash computer. True I have never driven until the engine stopped, but I would not expect an "empty" reading to show when there were so many gallons left according to the manual.