I have noticed of late im getting semi poor gas mileage. recently changed the oil put new plugs in and ever since im getting bad gas mileage. I filled it up On 11.5 gallons and im less than half a tank now and i have been 150 miles. I have noticed that if i drive the car and say its 3/4s full and park it my gas will will be below the 3/4 mark when i get back in it. I looked for leaky lines and cant find any leaks and nor do i smell gas. Do you guys have any ideas on what could cause the bad mpgs and the disappearing gas