2000 Chevy Malibu said tank was empty after filling up

A weak battery,or alternator will cause this.

I have the same problem in my 2004 Buick Terraza. It is the sending unit in the gas tank. GM wants around $600 to change it out. They say the tank has to be pulled, emptied and aired out to open the tank up to remove the sending unit. What I do is fill up after 300 miles and not worry. When I fill it, no matter if I fill it full or part way, it goes to empty for a while and finally moves up slowly after 1/8 tank is gone, it acts normal again. But don't trust it after 3/4 of the tank is gone. It will show 1/4 tank but will move to empty within 10 or 20 miles. Use the mileage reading to know when to fill it. I get a check engine light that comes on and will stay on for a couple of drives. I had it checked twice and was told it was the gas tank sending unit showing it was bad. $125 charge to read the check engine light, but I'd get it back if they did the work. Been that way for over 6 years. It was under warranty at first,but the dealership put me off until it was past warranty. :-( told me I had to leave the car for 5 days and get no rental replacement. I couldn't afford to do that so never got it fixed.