gas guage reading incorrectly

I have a 2001 Alero. Just did tune up, replaced coils and coil housing. My gas gauge is reading wrong. If I have a half tank and drive a few miles it drops to almost empty. It was doing this when the car was running bad before I fixed everything else and I thought I was just using alot of gas because the car was running so bad. But now it runs great and the gauge is still dropping like crazy! Is there some kind of sensor to replace?

Yes. The fuel pump assembly has the float level sensor. You may be able to replace the sensor separately but in any case the fuel tank has to be dropped down for replacement.