The Hero Honda CD100's mileage would see a slight fall in its values as the engine being quiet old now produces more friction inside its working area, making the wear and tear effect visible through the drop of mileage. You can use synthetic engine oil for reduced friction of its components and the drop in mileage can also be due to uneven carburetor tuning. You can check and clean your air filter, clean the carburetor and get your engine oil changed for a rise in mileage and performance. Even riding above 50 kmph gradually reduces the mileage whereas riding between peak hours in the city can be the second reason. Avoid trafficated routes, ride between 40-50 kmph and get your bike serviced at the given interval would definitely result in a rise of mileage.