The tank will probably hold near 40 gallons but you can't get that much in it if it still has enough to run and the truck is sitting still. I have a pump on mine where I can pump fuel into main tank from reserve while driving and with the truck jostling around I can fill the tank to where I can calculate some tremendous mileage on the next fill up if I just fill the main tank and truck is sitting still. You got ripped, need to call the weights and measures folks. bg