The mileage, or fuel efficiency, of a bike is primarily affected by factors such as engine size, weight, aerodynamics, road conditions, and driving style. The presence of an additional person on a bike can affect its fuel efficiency to some extent, but the exact decrease in mileage cannot be quantified accurately without specific information about the bike, riders, and other variables.
When two people sit on a bike, it increases the overall weight the engine has to propel, which can result in a decrease in mileage. The extent of the decrease will depend on various factors such as the engine power, the weight of the riders, the riding conditions, and the style of riding.
Generally speaking, it is expected that the mileage will decrease to some degree when carrying an additional passenger. The decrease may be more noticeable on smaller bikes with lower engine power compared to larger motorcycles with more powerful engines. However, the specific decrease in mileage can vary significantly depending on the aforementioned factors and cannot be accurately estimated without more detailed information.