ivee just bought a 2005 mondeo tdci lx,


It had a full tank when i bought it, been doin the school runs (10 mile round trip) and a 50 mile trip back from where i bought it and thats it, when i first drove the car it said 500 miles left, now ive done about 75-80 miles and it says ive got 300 odd miles left to go and it almost used half a tank of diesel?

any ideas??
Many thanks.