Almost 40 years
Today I drove my original owner 1970. Stopped after about 10 miles of driving and was outside my car and heard a tone noise. The tone noise was stronger around the gas door. I believe it was the pressure relief valve on the gas tank. To be sure I slightly turned my 70's unvented/sealed gas cap and the normal built up pressure began escaping. The tone noise stopped immediately. First time I had ever heard this noise. About 1/4 tank of 93 gas in the tank. One thing for sure, my 70's original sealed gas cap will pass inspection. Some things on these old cars amaze me when they still work.