Peugeot 308 using exessive amounts of fuel compaired to usual

get car serviced if not been done for a while.

Hi there my name is yorkie, I would like to ask a question about my Peugeot 308 1.6 hdi,I bought this vehicle from a Peugeot dealer in Leeds two weeks ago, taking it home to Sheffield going down the motorway I decided to try it I was in the fast lane passing everyone I looked down at my speedo and to my suprise the speedo was only registering 65mph I realised the speedo was running slow,when I finally got home and switched off I noticed the speedo wasn't zeroing but dropping below the zero mark, so when the vehicle is in motion when the needle reaches zero the vehicle is travelling 10mph, so when the needle is on 70mph the vehicle is travelling 80mph.Took it back to dealership but wouldn't accept it was faulty so at the moment we are at stale mate,left the vehicle with them and informed them I was not going to take the vehicle on the road with a defective speedo.I forgot to mention is only two year old and still under warranty,they refused to change it under warranty.Wondering what to do next have any you guys had this problem,looking for advise.