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.