1998 Dodge Dakota fuel problem

I have 98 Dodge Dakota 4x4 i got it three weeks ago. When i got it i was getting 17 mpg the next time i put gas in i got 14 mpg then the last time i got 11 and this time is not looking good either. I changed spark plugs and wires but it did nothing. It does not have a gas leak anywhere and the engine runs nice and smooth. I noticed that the vent tube was not on there so i put one on that goes from the tank to the filler throut and it didnt change anything.