Toyota 80 series fuel tanks.

Thanks Allyn, thats what I wanted to know. However, does the tank hang down noticably at the rear? Is it susceptable to being knocked or damaged?

Desert
Tank sits beautifully underneath and is far higher than your spare wheel would be currently. You cannot see it from behind at all.
The extra 20 litres or so didn't concern me as you don't need to fill the tank to the brim unless travelling far enough to need it, in which case you would have a jerry can with same weight anyway and it's really only 20kg isn't it. You will need beefed up suspension before this mod both because of the additional weight and due to the spare wheel and carrier being further back on your vehicle. I added polyair springs also but not really necessary.

the weight is far better being under the car than anywhere where it affects your centre of gravity ie. sitting on top of a roof rack. How often you fill the tank to the brim is entirely up to you. I fill mine every time but I live a lot further from nearest town that I would visit and use a fair bit of the stuff it holds. It will pay off eventually though, for example I was able to complete Gibb River including Kalumburu without refilling. We paid $1 per litre in Kununurra and the rest of my crew paid $1.50 at Drysdale and Kalumburu. I debated the issue over and over and in the end thought that if I couldn't make my mind up then I would go for it. Can't say as I'm unhappy.