I've got a '94 wagon that gets ~20 mpg in the winter and ~24 mpg in the summer, all highway too. The smell you describe is one that I've smelled too around my car sometimes after shutting it off. It is not consistent though. Could it be the seasonal gasoline quality? BTW, my wagon has 197,000 on it and oil that has the same color as honey after 3,000 miles.
Have you had the head gasket and wiring harness changed yet on your car?