Erratic fuel gauge

Functions as designed when tank is full. Indicated level falls rapidly to 3/4 within 20 miles and reads almost empty after around 80. I know I'm not using that much fuel even with my heavy foot. Sending unit looks newish (I've only had the car for a few years and was always like this) but am thinking of changing it due to them being inexpensive. Is there any adjustments that need to be made when installing a new one? Could I have another problem other than the sending unit?