Mitsubishi Expo Forum : “Why won't my car drive in Overdrive?”

I would suspect it is a bad tranny or they put a 3spd in you 4spd car, or something got streched

I have the same problem with mine.  The overdrive is the 4th gear and seems to fail first.  You have a choice:  You can live with it or you can spend around $2000 to rebuild the tranny.  There is kind of a third choice.  You can change the fluid and filter and see if that helps.  If you change the fluid make sure the fluid is replaced with fluid that meets the ATF+4 spec (This is for Chrysler, Mitsubishi and I think Hyundai trannys).  If you live with it you gas milage will never be as good again, but it will be OK.