This is just my opinion, so keep that in mind -

Since this is just a winter beater, it might be cheaper just to live with the 2mpg economy penalty and move on to something better next year. Throwing parts at the problem can get expensive fast - way more than 2mpg over the course of a season. Your driving style and your mood can affect your fuel economy far more than you may realize, if you're used to diesel, you may be expecting more than that car is able to deliver.