Only about thirty for me. I remember a NOVA show about this, "The Invisible Flame", from the early 80s. IIRC there was a town somewhere out west (Colorado?) where the vehicles had been adapted to run on either hydrogen or gasoline. The switch back and forth was accomplished by only throwing a couple of valves under the hood. Never heard the outcome of that experiment..
The Invisible Flame (1979). NOVA S6E6. You can rewatch it on Hulu.