Mileage varies with driving conditions and style, AC use, size of tyres, gearbox / differential combination and obviously state of the engine particularly the nozzles and diesel pump. 8 is on the high side so firstly you should find a decent mechanic and diesel laboratory in your city and get your pump and nozzles checked. These can be replaced/retuned to give better mileage but there will be a tradeoff with acceleration.