@ undefineme3: Of course, the mileage increases as all gears work tall, provided same riding style.
Its very obvious. Even in stock bike, 5th gear gives better mileage than 4th ...
Similarly, when you increase overall gearing, engine runs at lesser rpms, and bike travels further for every drop on fuel.
Although, if you are changing more than 2 teeth overall (positive or negative), then you will have to get another chain (with more/ less links, as per change).
Below 2 teeth, Chain can be adjusted. That's why I recommend changing front one only, as far as possible, as Chain is almost never changed.
But, you can change both sprockets too, if they fit.
--
@ Prabhakar: Thanks. More from TVS will come up soon ...
--
@ insanebiker: Yes, according to Rolon website, they supply Bajaj with Chain-Sprockets.