Propane conversions are common in Europe and with propane, you can run a dual fuel setup. There are kits which have tanks that are the same shape as spare wheel wells so that you don't lose any trunk space. I have a 75 Jaguar XJ12 that runs on propane. I'm thinking of converting it back because it's not a dual fuel setup. It has a huge regulator that bolts into the intake on each side which is how the propane is fed. Much like throttle body injection.

I think propane for car use (not barbecue) is about $2.90 / gallon.

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With best regards

Al