I'm under the impression that the usage is more like 20% bacause it has about 20% less energy pre unit than gasoline. I suppose if you just want to be a lead foot then you'll use 40% more to get back to the same power output

so when gasoline is more than 40% more expensive than propane it becomes cost effective. If there ever is hydrogen or methane or whatever then you probably won't need to change much to use that fuel.

Since the there may be less taxes, you could get a 10 - 20% discount off the top, so that means gasoline only needs to be 1/4 more. TAX FREE IN AU till 2011!

I was looking and for example in the UK:
unleaded 95: 87.2 / l
diesel : 91.1 / l
super : 89.5 / l
LPG: : 34.0 / l

95 octane is 2.56 times more than LPG. this is also because LPG doesn't have 66 - 71% in taxes added to it.
source: http://www.theaa.com/allaboutcars/fuel/

A LPG kit in the uk costs about 350 for a datsun.

all the parts for an LPG kit don't weigh 50 lbs. ( when tank is empty )

something to consider...

Hydrogen power looks sweet. Endless supply to run off, zero polution and any vehicle can be modified to suit running it.

BMW have a system fully developed, it is expensive at the moment like all new technology, but will be achievable within the next ten years.

In Colac, Victoria, Australia. We pay $1.33 a litre, Lpg 72 cents a litre 43 percent of these fuel prices goes to fed's govement hand, 18 percent goes to local state gov hand, 1 percent goes to service stations & and the rest goes to the (Oil) companys who is laughing all the way to the bank with smile on its face and who;s crying fowl, we are and doing nothing fix the problem. Sounds like when leaded fuel was killing us but oil companys blocked anything being done to fix the problem then & 80 years later when our children being poisened, something was done then. Sadly Bruns