spark plug problems and fuel usage

[fiesta8u]

hey guys how are you all ? well somethings going on with my fiesta, my spark plugs foul every up to 500kms what the hell could be wrong ? im using bosch platnums when i change the plugs it goes hard then you can feel it and hear that the spark plugs start fouling about 200 kms later and then just becomes really bad at about 500kms why is it fouling plugs all the time ? and also my fuel usage is just stupid this week i only got 180kms out of a full tank, yes 180kms yes 180kms its not a typo, when i start the car and when its at idle it smells like fuel, the car only has 26000kms on it, ever since about 10000kms ive only been getting about 280kms to a tank, all that i have got on it is a muffler similiar to a cannon, ive taken it to ford about the fuel usage a million times but they always want to blame it on the exaust, for the spark plugs do use think its because of something in the engine or are they crap plugs if you think its coz of the plugs what plugs do you recommend ? and i know that foul plugs can use more fuel but even when i put new plugs i get pretty much the same, what could be the fuel usage reason ? air filter is a little dirty as well but that wouldnt do much, thanks.