AHHH!! 11mpg, i remember those days when my 300E ate that much gas, I semi got it fixed. I guess it's about 18city now. One of the BIGGEST contributers to it was that there was a vacuum leak. Really I would check that first, my leak was under my exhaust manifold, some tube, don't recall off hand. Thats really your best bet. If it's not that, i'd go with just a normal tune up to get everything running correctly. It really does sound like a vacuum leak tho, but if the car hasn't been tuned up for awhile, i'd just spend a weekend doing all that.