lots of things to weigh up...
I used the calculator on that thread....
I put in for a typical alfa 35mpg, a price per litre of 144c for petrol and 99c for lpg, and reckon have to be doing 20k+ miles a year to justify the cost (savings of €650 for the year)... Even then it takes ~2yrs (£1000 ~= €1250) for it to pay back the investment and in the meantime your stuck with either no spare wheel (important on Irish roads
) and/or lack of boot space.
In addition - cos it's hard to get here, you have to factor in a % use factor.. The calculator assumes you move completely 100% to running on LPG, so if you only get to use it say 40% of the time it just doesn't become economical in this country as your savings are now only €260 per year - i.e. 5yrs to pay off the investment.
In the UK - moving to LPG makes sense. It's not taxed as much and is about 40-50% of a litre of unleaded. Here it's about 70% of a litre of unleaded, so the savings aren't so great. Also, because there was a push of LPG in the UK back in the mid 00s they have the infrastructure in place - we do not.
It all comes down to your situation and usage.