My car has dropped from the normal 22 mpg to anywhere from 16 or 18 mpg depending on the day. My check engine light is on, and my gascap light is on. However I have tightened the cap and it doesn't appear to be damaged or anything. I am over the 36k warranty (40k miles), so it will be $115 for the dealer to "diagnose". My main question is I need to go out of town tomorrow, just 3 hours away. My car seems to run fine. Is this something I can deal with when I get back? Thank you.