1996 Nissan Maxima Speedometer and RPM guages dont match. WRONG GEAR!


I am new to car talk and dont know a ton about cars but i do know that my old study 1996 Automatic Nissan Maxima has her check engine light on (that thing has been on for years on and off- cant seem to get it off) but more importantly shes not in the right gear when I’m driving.

I’ve had this problem several monthes ago and my Taxi Mechanic thought it was the computer giving her the wrong signals so he changed it for a better used one. Now its happening again. I’ll be doing 60 and the RPM will be at 2- no higher. Any ideas?? I fear that i’m going to wear my transmission down fast. Shes almost at 200,000 miles but looking to get as much as i can!