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High-density polyethylene, metal tanks have not been used for years; fiberglass is not practical.

Metal tanks in an accident used to break at welds joints, allowing fuel to leak out. Plastic tanks can take quite a hit before they break; thus less likely to leak fuel. In addition, if there is any moisture inside a metal tank, it will eventually start to rust, allowing rust deposits to plug the fuel filter, plastic tanks can't rust. (One of the many reasons why fuel filters do not need replacements today, like they used to years ago.)