I think my alternator might be bad in 89 Oldsmobile Cutlas Siera?

You should be getting ~12v with the engine off and ~14v with it running. If the alt. is not functioning properly, battery voltage will drop as you drive. Low voltage means then engine cooling fan will turn slower and it may affect your gauges (such as your speedometer). Take the alternator out (not hard at all on your car) and take it to a auto parts store and they should be able to check it for you. If it's bad, then you're already there to pick up and new one and no core charge...