Why has the mileage of my Yamaha SZR decreased?

Since the Yamaha SZR is an older model, it is likely that its fuel efficiency has decreased due to the build-up of dirt and debris in the engine. As air and fuel are not burning as efficiently as before, this can lead to a decrease in mileage. It is important to ensure that you regularly maintain your vehicle to keep it running at its best. This may include replacing the air filter, spark plugs, and fuel filter, as well as checking the condition of the exhaust system. 

Additionally, the fuel injectors may need to be cleaned or replaced to help keep the engine running efficiently. If you find that your vehicle is still not performing as expected, it might be necessary to have it serviced by a qualified mechanic.