Well, could be the dealer filled the diesel with petrol, those things happen you know.
i just checked and my little baby i30 CRDi "Betsy" will be one on April via rego lable, she has got 26,000 trouble free k's up, has averaged around 5.2 for her life thats also in Sydney traffic and up and down the coast and out into the HOT & Wild West of the state ( rough roads) her best to date has been 4.4 that was when she was almost new, its how you drive that get's economy not how many k's you have up. she is runing as good now as the day i picked her up. i'm happy.
cheers