You'll occasionally still see one of those propane conversions on the road. I seen a 70s van filling up with propane a few months ago. From what I understand, mileage wasn't quite as good and most conversions had less power, but they worked and propane is usually notably cheaper than gasoline. For some reason I seem to think they didn't start well in real cold weather, kinda like an older diesel, but that may just be my faulty memory. I've never actually used a propane fueled rig, so it is all hearsay. By the way most forklifts run on propane

I've heard of some farmers who have natural gas wells on their place converting equipment to run on natural gas. Only way I would think it would be economical.