Not that it matters...too many people here trying to pinch someone else's pennies for them, but the savings part, again like it's anyone else's business, can be real.
If you live somewhere that fuel prices are consistent, or don't travel through high price areas from your low price area then significant savings at the pump may not be huge. But on the flip side, if I had a 60gal slip in my diesel and used that truck to travel to eastern wa frequently (I don't, as I have a work truck), I'd save a buck a gallon or more most times.
$100 a pop, only take a short while to pay off that tank.
Fuel mileage difference with an extra 500lbs in the truck is insignificant.