I have a 2009 Cobalt LT. I noticed a couple months ago how I had to put gas in my car more and more frequently. It was time for an oil change so I figured that was the cause. Now it went from around 28.9 mpg to 16.1 and its dropping by the week! Ive changed the air filter and keep my tires inflated so I need some help. I'm in high school so I don't exactly have a lot of money to throw gas in my car, any advice??