Update (Feb 2010): lean burn engine installed for more MPG

In the late summer of 2009 at 512,000 miles my engine and transmission were wearing out, so I had a lightly used transmission and lean burn (VX) engine installed in my car.

So far, the new engine is averaging 7 mpg better than the old engine. This engine doesn't get the same mileage over a broad speed range that I saw with the previous engine, but instead shows the steady decrease of mileage with increasing highway speed typical of most cars. In the summer on a level road the new engine gets 100 mpg at a steady 60 mph and 120 mpg at 40 mph. It has tank averages in the upper 60's in the winter and upper 70's in the summer. I have had it remain in lean burn on a level highway up through 91 mph and estimate that it should be able to remain in lean burn up to speeds slightly above 100 mph.