91 Fuel Gauge Problem

Heres the thing, I have a 91 5.0 Notch LX and all of my stock gauges work, except for the fuel, it reads empty all the time. I was told it may be the sending unit, but i have no idea wat to look for... another question i have is how many liters is the tank, 68?? or sumptin like that, I have been getting gas based on my trip counter and looking at my KMS, how many KMS should a full tank of gas get me.....