Gas gauge only goes up to 1/2 tank when we will it up but drops at a normal rate, needle won't go under the 1/2 tank mark unless the gas is below half a tank, anybody know why that is? Would it be the fuel sender thats giving a false read on the dash? Its not doing anything performance wise, so its not a major issue but I'd like to be able to know it really is a full tank of gas.