Dodge Dart gas cap.

Will_H#24 said

Thanks. I desperately need a new seal for mine, but alas no one sells them

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Go buy a new 60s Ford pickup gas cap .

Drill out the center allowing the rubber seal to come free.

It will fit over your stock cap and tuck down under the stock center.

I did this on my 67 and it has sealed very smell for 7 years

Like this one

NEW gas fuel cap 1949-59 Ford car, 1951-1971 Ford pickup 8A-9030 | eBay