I found this thread to be really interesting. I currently have a 2001 110 TD5 which the previous owner fitted dual under body long range tanks to. (both around the 40-45 liter mark).
On a number of outback trips I would regularly travel over 1200km without any problems (never faster than 80-90km/hour due to terrain/danger/risk/fuel efficiency). Even with sections that used low range etc with load.
What I like about the under body tanks is that the balance of the car is pretty nice when you are fully fueled and loaded all the weight is not down the rear of the car. What I do not like is that they require a fuel pump which drains the extra tanks into the main tank. I have yet to find a reliable bush-proof pump. The original owner fitted Holley Red pumps, fairly cheap and mostly reliable but the seals constantly go in them and leak diesel all over the place. It seems that they tend to bust every time I go to the outback. I then replaced them with Holley blue because they are meant to be higher flow and better quality bits. Unfortunately they have never really lived up to my expectations (of lasting 3 years with at least quarterly use). I have been told that the pumps need more regular use to keep the seals wet so they do not perish. (you may ask why I do not find another pump vendor.. that is because the mounting brackets etc all fit the holley and it's a nicely done job.. getting in and replacing the pump with something else would require new brackets and work which I didn't have the motivation for.. (btw this is not a winge about the pump's they are relatively cheap ($115 inc postage) it's really just giving some more context)
Now that I am looking at upgrading to a 2014/5 Defender I looked at the various options again and so far I couldn't find a similar product that was cost effective. I have been looking at doing the forerunner under rear arch tank as a possible substitute but it doesn't give me the millage that I have now. I like the fact that it is gravity fed after the problems that I have had with the pumps on the existing units.
Keen to know how other people have felt with the different styles of long range fuel tanks and what their experiences have been.
