Mechanic Q: Car shudders and mpg dropped?

My car shudders only between 45 and 55 mph. It doesn't feel like the tires, but more like an engine misfire. This started the same time my mpg dropped 25%. My low oil light started coming on at the same time, but my oil level is fine. Sometimes if I drive for awhile and then turn off my car the light will go out. There is also a pretty intense smell when I open my window on the highway. It kind of smells like concentrated exhaust.