My experience with this issue was that the float part of the sender unit was partially full of fuel. It must have sprung a small leak and the fuel weighed it down and the gauge read low. Very easy to removed the sender unit but do it with low fuel in the tank, float clips out easily. Also the float is the same as those on a mini (at least it was in my 85 XJSC). Gauge reads much higher although not always to Full. Regards Martin