Any suggestions as to what may be causing a sudden, small, but significant drop in fuel economy? I do a fair amount of steady highway driving, generally setting the cruise control between 70 and 75, and I had routinely been getting about 31+ mpg ('95 325i, 5-speed). Since November, however, over the same routes, I've only been getting ~29 mpg, a small but noticeable a 6-7% drop. The car has been running fine and smooth, the idle behaving as it should, tire inflation at 32 psi, oil changes at regular intervals, air and fuel filters replaced last spring. I've heard that colder weather adversely affects fuel economy (longer time to reach operating temp., denser air), but it really hasn't been all that bad where I live; also, winter formulations to make gasoline more volatile in winter can lead to decreased fuel economy, but I rather doubt all that much.
Again, this came about suddenly, which leads me to think that something mechanical went out of whack, as opposed to a gradual decrease over time. Possibly a vacuum leak? Thoughts, suggestions appreciated.