26-27 mpg doesn't sound very rich to me. I have been struggling for the past two years to improve the mileage in my '90 240 with an automatic. It was getting 17-20 mpg around town. What I found the cause to be was exhaust leaks upstream of the O2 sensor. These leaks let in oxygen into the exhaust stream which is picked up by the O2 sensor making the ECU think there is a lean condition. After plugging multiple exhaust leaks I am happy to be getting 24-25 mpg around town (my car rarely sees the open highway).