97 sonoma gas gauge

I was wondering if its just an electrical problem or not but every time i fill up my truck and its on empty it only takes around 12 gallons and its an 18 gallon tank.I just had a new fuel pump put in and sending unit any ideas would be helpful.

The fuel level sender is probably out of calibration. When replacing the module, the level sensor arm can be bent very easily if you're not careful. Tricky to recalibrate it, you might be better off to get another pump module.