Hero Motocorp — poor mileage

Hi hero
My name is hari lal i have the bike hero ismart 110 when i brought it the dealer says i got the mileage above 80 or 85 km/litter now my third servce is finished and bike runs about 6500 kilometers but i got only 55-58 km/ litter i ride bike only in an average speed 40-45kms/hr but no improvement what i do for better mileage please consider my complaint
Yours faithfully
Harilal