240 gallon tanks only holding 170 gallons

What year is your truck?
The fuel level sensor can go bad.

 

Your fuel gauge could be off, or whoever installed it placed it higher than normal. Try using a fuel measuring stick to find out how much fuel you have left.

 

tanks usually hold 90% of the advertised amount which would be about 215 gallons.

do you measure once the pump stops? Or do you raise the pump from the tank and fill it up until the fuel is about 5 inches below the top of the neck?
The reason I ask is my tanks hold 200 gallons and only way I can get close to that number is if I raise the handle and fill it up until the fuel reaches the top. It’s about another 20-25 gallons per tank.

 

My 2013 WesternStar had that happen, the gauge gradually read lower and lower over a few weeks.
Take a look at what the gauge does when you first turn on the key before engine start.
Normally it will sweep all the way full and all the way empty, before settling on a reading. If it still does this, the gauge itself is probably fine so the sensor in the tank is most likely the issue.
In my case, it only swept about half way up, then swept well below empty. So I replaced the gauge and it worked fine after that.

 

What you are referring to is not related to the tank capacity. If you tank says capacity of 200 gallons then you will be able to get 200 gallons in the tank.
What you are experiencing is the nozzle shutting off as the fuel back fills into the zozzle. Raise the handle and the tip is out of the fuel and can flow.

 

You need a certain amount of fuel left in the tank to avoid drawing air.