Hard to comment on the implications of something that has not yet happened
. I changed from a P2 XC70 D5 to P3 XC70 3.2 petrol (6 cylinder) with a Prins LPG conversion. It does everything the D5 did although the power is higher up the rev band. In terms of fuel consumption, it gives about 19 MPG, but I pay between about 50 and 60p a litre which is approx half cost of petrol so it's roughly around the equivalent of 40 MPG which on a par with a D5 auto. Main downside is its in a high road tax bracket (£535) but if you found a P2 before the date when they changed the classification, then it becomes a more viable proposition. Best option is to find a car with LPG already fitted as it typically takes around 18,000 mikes before you recover the cost of the installation. There are a limited number of factory-fitted Bi Fuel conversions, but I believe they're pretty old tech and many folks rip out everything apart from the tank and fit an aftermarket system.