Great info here on this thread.
About 5 years ago I went on an amazing trip through Utah and Nevada with some other 80 series cruisers. One of us had an auxiliary tank. Guess what, he could only go as far as the guy with the least amount of gas. That was an interesting observation. We chatted about that around a camp fire. Basically he said it's great and loves it, but really it's only a benefit when he is solo. if he is with a group, there's no advantage to him if everyone else is running out of gas.
The entire trip the rest of us were hauling jerry cans and dumping them in on occasion, but he was hauling more water and food!
This is exactly my point, just because I have an extra 15g (oem subtank) carrying cap, doesn't mean that I'm always going to utilize it. I have to stop at the gas station with everone else anyway, LOL!!
However, one pro of having 40g of fuel is that I can fill up at my local Costco for the cheapest fuel (usually) then go to the mountains, return home with plenty of fuel to spare. Whereas, the other rigs have to fill up with expensive fuel on the return trip home after wheeling. I have to purchase premium for the turboe'd 80.
As mentioned alreay, when I'm solo is when I really notice the "pro" of having so much fuel carrying cap. On trips when others are tipping in their jerry cans into their main fuel tank, it's nice to just press a button and keep driving.