Harley-Davidson fuel tanks are typically made from steel. Steel offers durability, strength, and resistance to impacts, which is important for a motorcycle component that is exposed to various environmental conditions and potential hazards on the road. Steel fuel tanks also provide good corrosion resistance when properly finished and maintained.
These alternative materials can offer weight reduction benefits or specific design aesthetics. If you have a specific model in mind, it's always best to consult the manufacturer's documentation or contact a Harley-Davidson dealership for accurate information about the materials used in the fuel tank construction for that particular model.