I just added a 30 gallon Transfer Flow fue tank tool box combo to my truck. Here is how my reasoning goes. Living in the rural west I have a self imposed rule of never letting myself get below a 17 gallon cushion of fuel especially when off road. Because my truck only has a 34 gallon tank that means I only had 17 gallons of usable fuel. Since adding the 30 gallon aux tank I now have 47 usable gallons without dipping into my fuel reserves. Way cool!!