2001 Pontiac Montana Fuel Gauge?

2001 Pontiac Montana,the fuel gauge always says its full, when you first turn the key it will go up so far (which I assume to be the correct amount) and then after started goes to full. I don't know how long its been doing it and personally have only drove the van once and that was home, as my wife bought it from her family, I generally don't buy Pontiacs as I've never had any luck with them but wondering what some options I have to correct the problem????? I assume a new unit in the fuel tank is where I'll end up overall but in the meantime I'd just like to know its keeping good gas.