snow melted enouph got 91 fire bird out of back yard and took it for a drive around the block .i notised that the speedo was off about 15 or 20 mph off called my stepson and he told me the guy he got the car from changed it , so i asked what they did with it ,file 13.so i called gm parts dealer and he toled me to check the trans. gear ,it waw the right one for the car .then he said thespeedo could be the wrong one?could this be ,and what do i do now!