Gas mileage falling drastically from 30mpg hwy to 18mpg

replace your spark plugs with oem.

One or more of your Oxygen (O2) sensors are probably bad. Unfortunately, O2 sensors and car computers aren't very smart. As long as an O2 sensor is sending a signal to the computer the computer assumes it is working correctly. If the O2 sensor is sending faulty information but within the design parameters the computer assumes it is working and will continue to dump more fuel into the system causing your MPG to suffer. Over a long period of time this could also ruin your catalytic converters. If/when you do replace the O2 sensors make sure you use a factory part or a very high quality part for the repair. I would NEVER use a Bosch O2 sensor since I've had them fail when brand new. I think there are four O2 sensors on this car and there is a way to check each one to make sure you only have to replace the bad one(s) but if you aren't mechanically inclined it could be a hassle.