That is a 40 gallon tank .......actually 43 but you must allow for expansion.
Was the tank completely empty?
Could it be there are compartments inside and it is not flowing ac across?
The most likely answer is the vent is clogged or not properly vented.
I had an old truck that I could only get about 20 gallons in the 32 gallon tank and it was the gauge off, it would read empty but still had about 12 gallons in it. I found out when I was able to make it over 100 miles on E with a big block.
Thanks for the replys. I thought it should hold more I have a fuel pressure valve that went out so I will drop tank and check the vent and inside of tank for blockage. It sure makes it hard when towing the boat and you have to start looking for gas stations every hundred miles. Thanks again.......