Here's how my car performs on LPG (disregard the earlier claim!):
Refuel date___ km traveled___ litres used___ lit/100 km
19/2/09 ___ 156 ___ 25.72 ___ 16.51
22/2/09 ___ 181 ___ 26.81 ___ 14.81
Driving to Adelaide
22/2/09 ___ 536 ___ 52.58 ___ 9.81
23/2/09 ___ 715 ___ 75.31 ___ 10.53
Gauging from the last column, the fuel consumption is more or less on par with what I get out of unleaded petrol, both for suburban driving as well as for long distance driving. Hence the savings with LPG is more or less dependent on the price of LPG with respect to unleaded petrol.
LPG price varies GREATLY between pumps. In Melbourne, most pumps are currently charging between 49.9-52.9c/lit, while the lowest goes for 36.8c/lit. In Adelaide, all the pumps I saw were charging 62.9c/lit (great cooperation between retailers?). Along way to Adelaide, the prices were all consistently higher: from 69.9c/lit to a whopping 73.9c/lit in Keith! Interestingly, petrol price does not vary much from Melbourne and all the way to Adelaide. Even in Keith where LPG was 73.9c/lit, the unleaded petrol was 128.9c/lit, which goes to show that LPG prices appears are highly out of control. Does "profiteering" cross your mind?
(added on 3 Mar 09) If you wish to make a complaint about unfair pricing, make a call to the ACCC - Australian Competition and Consumer Commission. I just did. The telephone number is 1300 302 502.