There is a good reason why LPG is not that popular any more. For starters I think the pricing is closer than you make it out to be. You shouldn't be using premium price to compare plus you need more liters of propane to travel the same distance AND it's hard on the engine, especially valves, loss of power, where do you put the tank? etc. etc.

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1988 325is (purchased new) sold
2004 X5 3.0 2005 X3 2.5
2008 X5 3.0 (new to me)