LPG isn't equivalent to double the "standard" mpg either.
You normally get the equivalent of about 70% on top of the normal mpg.
My Saxo does 40-45mpg on petrol, but I generally only see 35-38mpg on LPG, I believe this is because the car produces slightly less power when on LPG. It's also worth remembering that you need petrol to get the engine warm enough to run on LPG, this means running it on normal fuel through the cold map, for at least the first few miles of your journey. Might not sound like much, but on something like a 750i it wouldn't be negligible.