Someone asked if the extra weight of the gas would cut down on MPG, yes it does, but you don't have the extra weight in the spare tank normally. If you know that you are going to have to go someplace where the nearest gas or diesel source is 100 miles from say a campsite & you plan on driving around exploring while at the campsite, then you may want to full every container with gas or diesel before leaving to go camping. On the other hand, if you are just going to be close to a fuel source, you would only need your OEM tank.
I on the other hand, live in San Diego Ca. & gas is very expensive, where the gas in Yuma Arizona is 40 cents cheaper a gallon. I usually go to Yuma once a week to meet a semi truck with a Wide Load, so i fill everything that i own with gas over there & drive the rest of the week doing local jobs on the cheaper Arizona gas ! When everything is full, i have something close to 80 gallons of gas. When i just have the OEM tank full, i get around 17.4 MPG, with everything full, i get about 17.2 MPG according the the EVIC
Here are the pix that i said that i would post
first i have to remove the soft tonneau
the 2 fiberglass strips have velcro on them, they are there to keep the tonneau from sagging when it has water on it