From personal experience, with my 1992 525 with GM4L30E Tranny: Do a fluid change and be sure you fill it with the correct level of fluid...drive it a day or two and then recheck the fluid level. This tranny is very sensitve to fluid levels and if you have too much your MPG will be down. In addition, clean your gear selector switch under the shifter cover. That too could contribute to low MPG and less than optimal performance if it's dirty. (One indication of this is that your LCD display may be missing some pixels.)