Help!! 1996 oldsmobile Aurora at highways speeds at around 70 my rpm is at almost 5500!?

I do not feel my car shifting while in D it seems like just press the pedal and go! I put the car in 3rd gear same thing. My gas milage is also shot to half! Don't kno I'm puzzled like my overdrive is kicking in or TCC isn't locking or Bad PCM! It all started happening after I got my water pump replaced car is saying low coolant and the damn thing is filled! To backtrack a little my coolant was overflowing from the cap and the engine was running hot before the water pump got replaced. Now the engine temp is fine stays around 200. Sorry to get a lil off topic but like I said help 70mph and 5500rpm help me thats a big no no help me please!