To get a good mileage, bike needs to fine tunned. Air filter, carburetor should be cleaned, valve in head in good condition, tappet are not too lose not to tight, oil in good condition of a good brand, chain sprocket in good condition (proper oiling), proper tyre air pressure, tyre should not be jammed a bit.
Small gears like first and second consume more fuel than third and the fourth. Lastly, a gentle pick from the rider not like to go for a race can increased fuel mileage.