Here's another idea to throw in... vacuum leaks. Last winter I was only getting around 16 mpg. I took it in to discover that there was a vacuum leak coming from the hose that hooks into the idle air control valve. Now I get around 24-25 mpg. Another symptom of vacuum leaks is rough idle. I can't guarantee that this is your problem, but don't overlook the obvious. By the way a properly running 300e should be getting around 20-25 mpg...