Around these parts, pretty much everyone who relocates the spare puts a fuel tank under there. The exception to the rule is a mate of mine, who pulled his BBQ apart and installed it on a pair of slides under the canopy on the back of his ute. It sits where the spare used to be between the frame rails, and simply slides out the back when he arrives.
For gas (petrol, in AU-speak) motors, it's popular to fit a donut/toroidal propane (LPG/gas) tank where the spare used to live since LPG has traditionally been much cheaper than gas/petrol (not so much these days though). The downside is that LPG isn't readily available in the outback, and no-one's yet been able to come up with a decent jerrycan for the stuff.
The simple option is a long-rang Aux tank, which can significantly beef up both the touring range and the weight of the vehicle. Unlike jerry cans, you can't take it out when you're driving around town. With gas/petrol currently about $6/gallon (roughly), having a couple of hundred litres onboard makes for an expensive fill-up. These guys are popular :
http://thelongranger.com.au/4wdfueltanks.html . ARB have a range of combo tanks, that will hold, say, 70L petrol and 30L water. If you don't need the range, having extra discrete water containers is never a bad thing.
The diesel guys (which is by far the majority of people) normally just fit a long range tank. Diesel-gas systems are popular here, so some will squeeze in a small propane/LPG tank as well for use in that situation. See here for the details:
http://www.dieselgasaustralia.com.au/
The most common reason that people will relocate an underslung spare here is to prevent the spare from being damaged under the car, and to ensure that you can get to it easily if you need to. IE, if you stake a tyre half way up a rutted hill, being able to lift the spare off the back can be a lot easier than having to jack up /dig under the vehicle to pull out the spare from the OEM spot. Also, to carry two spares for extended remote trips. As for me, the bigger tyre just plain wouldn't fit underneath, so it lives on my roofrack now.
I'm looking forward to seeing what ingenious ideas people come up with!
Cheers!
Matto
