Gas tank ?

I thought the gas tank in my 76 Bronco was a 14 gallon tank like the Chiltons book says.My gas gauge hasn't worked properly for quit a while,so I would fill the tank when the gauge was close to empty. Well, I let the tank go dry or close to it and noticed when I filled the tank it only took 11.5 gallons.I know some Broncos came with 12 gallon tanks. Did some 76ers come with 12 gallon tanks or is there another issue?

tank

Don't know for sure but, the orginal I took off would ony hold 11.5 empty.