When filling a motorcycle gas tank, it's generally recommended to fill it to the recommended level indicated by the manufacturer. Motorcycles usually have a fuel gauge or a visual indicator on the fuel tank to help you determine when it's nearing full. It's important not to overfill the tank because it can lead to fuel spillage, which is not only wasteful but can also be a safety hazard.
Overfilling the tank can cause the fuel to expand with temperature changes, potentially resulting in fuel leakage or damage to the evaporative emissions control system. Additionally, leaving some space in the tank allows room for fuel expansion. It's best to consult your motorcycle's owner's manual or contact the manufacturer to find the recommended fuel level and any specific guidelines for filling the tank.
Remember to follow proper safety procedures while refueling your motorcycle, such as turning off the engine and avoiding smoking or using open flames near the fueling area.