Hi, My honda cbr 150 mileage has suddenly dropped from 42km/l to 30km/l.. Can i know the reason? I have properly maintaining tyres and chain,,, Even im using the same company fuel

 

Many things can effect gas mileage: spark plugs, spark plug wires, air filter, motor wear, dirty injectors, etc. I'm not an expert on bikes, but I'd check and possibly change spark plugs and wires and air filter at a minimum if you haven't done so in a while and see if that helps.