If you are talking about a tractor as in tractor trailer, most of them hold a minimum of 200 gallons.
My truck came with a 40 gallon tank. the extra 100 would be nice on long trips. Wife would never get to stop and pee.
Mine is specifically set up as a gooseneck tow rig. A 40 gallon tank down and a 66 gallon tank behind the cab..sitting in plain sight in all its diamond-plated glory for every "SS" to see.
btw: WTF is an "SS" and why would they care about my fuel tank?