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I think the answer depends on why you want to convert. If it's purely monetary then work out the cost of conversion and the mileage you need to do to recover that. If it's ecological then research the figures and decide.
My previous Discovery was a 3.9 V8 in which I was doing about 800 miles per week so a conversion to LPG made monetary sense. I didn't really consider the environmental effects.
I didn't see much change in fuel consumption (still managed 16/17mpg) but with LPG at half the price of petrol the savings were impressive. The vehicle actually ran better on LPG ~ smoother, more responsive ~ than petrol.
I went for a single point conversion (now I would go for multi-point) with 2x40l tanks mounted in place of the normal petrol tank and a small (25l) petrol tank mounted in the rear wing area. This meant I could still greenlane and didn't lose any boot space.
I could probably dig up some photographs if you are interested.