Yeah, about 20 mpg on petrol, and just over 13 mpg on lpg.
As Deasy said, it's the cost of running the vehicle, not the vehicle worth. The conversion should pay for itself in a year (actually, this depends on mileage), and should add some value to the car (£500 - £1000), depending.
How I work out if its worth an LPG conversion.
I get LPG at 69.7ppl and get 13.4 mpg on this, so in 20,000 miles it costs me £4749.514.
I get Petrol at 1.339 ppl, and get 20 mpg on this, so in 20,000 miles it costs me £6087.044.
So the difference is £1337.53
A decent LPG conversion would probably stand you at £1800, and would likely add £500 to the value of the vehicle, so I would need to drive 20,000 miles to recover the cost.
TBH, I would be very unlikely to keep any car that long. I change my cars as often as I tax them, so it wouldn't be worth it for me.
With that in mind, I paid for a vehicle already converted. So any depreciation will be just from the vehicle, and not from the conversion.
So then I am left with above calculations, which puts me at 25.74 mpg when comparing with Petrol, or 26.70 mpg compared against diesel.
Except that isn't right either, as if I were to run a diesel, I would be running 50/50 diesel/veg oil. Which means I would be looking at a 22.0033 mpg equivalent.
Which is probably lower than I would expect if I were running a diesel, so its costing me more to run LPG than to run my preferred veg/diesel option, but then again, I am driving a big V8 petrol, so it's worth it.
Of course, I do have a 100 mile daily commute just now, so I just bought a 1.9 diesel Polo for this. The average consumption for which, according to Fuelly, is 55 mpg, so with my 50/50 should be equivalent of 66.75 mpg. Then again, my 335d averages 5 mpg more than the average because of the nature of the commute (all dual carriageway), so assuming I can average 60 mpg, this is the equivalent of 72.81 mpg. That'll help make up for the 26 mpg equivalent from the RR, and 37 mpg in the 335d.