I have 01 buik century that my speedometer is not workin right and when my speedometer goes down my car shiftsdown i replaced speed sensor but didnt help what can i do

The speedometer malfunction and downshifting could be related to the speed sensor. Thus a question, did the replacement of the speed sensor fix the downshift issue? If so, the speedometer has what are called "servos", which are small motors receiving signals from the PCM via various sensors (such as the speed sensor), and use them to drive the needles in the various components in the speedometer cluster. These go bad after time, and will require a rebuild of the cluster by an expert. Odometer fade (resistor failure) is another common issue in the cluster as well. If the speed sensor didn't fix either item, a pronounced downshift could be a symptom of several things, but the failure of the pressure control solenoid in the transmission may be the issue, and could have a pronounced affect on the speedometer read as well. Hopefully this is not your issue as this repair is not easy (removal of the front driver's side wheel assembly and part of the transmission), and quite expensive. Good luck.

My 1998 Buck century speedometer is not working at all and my car will not shift out of low gear

So I replaced out out speed senior didn't help so I think I need to replace the input speed sensor any help please been stuck for three days at a dang Walmart parking lot with no help what so ever I need any help of advice I can get thanks you all