i bought a 97 cadillac sts?

[Cierra Bradley]

i bought a 97 cadillac sts. when i got it about two weeks ago it had 146k miles on it. two weeks later it jumped to 235km on the dash board.. what is the cause of this? i also notice the speed goes up to 80 miles per hour and i know for a fact im not going 80mph.should i get rid of it? the car runs great but it needs a belt and transmission filter change. what should i do?
thanks yall i got it.. really appreciate it