interesting points on the costs involved.
i worked it out a couple of weeks ago for my 3.5, over the 6000 miles or so i have done in it, i reckon i have used about 2800 litres of gas, approx 616 gallons, and at an average of 52p/litre, thats £1456. Now if i had bought that much petrol, at say 105p/litre, that's £2940 - a saving of £1484. Even with the reduced efficency of LPG over petrol, i have still saved a huge amount.
to put it another way, it cost me £25 of LPG to get as far as Swindon the other week with a trailer on the back, it cost nearly £60 in petrol to get back.....