The fuel tank capacity of a Harley-Davidson Road King varies depending on the model year and specific configuration. However, in general, the fuel tank capacity of a Road King is typically around 6 gallons (approximately 22.7 liters). This capacity allows for a reasonable cruising range before needing to refuel. It's important to note that fuel efficiency can vary based on factors such as riding style, terrain, and load, so the actual mileage per gallon may differ. For precise information about a specific model year or variant, it is recommended to consult the owner's manual or reach out to an authorized Harley-Davidson dealer.