KNOW WHY AUTOMATIC TRANSMISSION VEHICLE GIVES LESS MILAGE THEN MANUAL TRANSMISSION VEHICLE !!

You might have wondered why 100cc scooter gives less mileage than 100cc manual transmission bike. Think about it where a 100cc scooter returns mileage near 40-50 Kmpl, an average 100cc motorcycle provides mileage over 60 Kmpl. The answer to this question lies in the transmission used where the scooters use automatic CVT transmission, while motorcycles use a proper 4 or 5 speed manual transmission.We will discuss about the reason behind this in this article.

Lets start with the purpose of a gearbox. The engine of a vehicle runs at a certain rotary speed or revolutions per minute ( RPM ). The required speed of revolution of the wheels is usually much less than that. For example, a typical 100cc bike engine can rev upto 6000rpm, but if the wheels ( 18 inch dia ) of a typical 100cc bike move at that speed, it would reach a velocity of 518 kmph!
So basically the gearbox is the intermediate mechanism which allows the engine and wheels to run at different rpm. 

The scootys use a system called a Continuously Variable Transmission ( CVT ), which in its simplest form is just a a cone pulley system with belts which allows for automatic adjustment of speed. The 100cc bike on the other hand has a manual gearbox, which requires you to manually select an ideal gear ratio for the given speed. Usually bikes have 5 gear speed ratios to choose from nowadays and they are thus called 5 speed gearboxes. 

Fuel economy depends on your RPM. The lower the rpm, the better fuel economy. Bikes with gears allow us to shift into higher gears and by doing so, the rpms decrease.At a given speed (say 40kmph), when in higher gear (4th gear), the bike has lesser rpms as compared to it going at same speed (40kmph) in a lower gear (3rd).In a gearless scooter, the rpms just keep building up continuously. So it has lesser fuel economy. 

Also, another major contributor to this is the fact that gearless scooters have a centrifugal clutch. In such a system, as the speed is building up, the centrifugal force acting on the clutch increases. Thus, it always remains partially engaged (detached from the input shaft). This translates into power loss and thus lesser fuel economy.  

 
The clutch is not controlled by the rider, it works automatically, and the clutch is also a pretty inefficient unit which draws its power from none other than the engine. This results in more loss of power and to make that up in terms of performance, the efficiency of the scooter takes a hit.

But most consumers don't mind because of the convenience of not shifting gears manually seems a fair trade-off. However, automatic transmissions are getting more high tech as we speak, and very soon this difference in efficiency will be negligible.