OK I Have a 1990 Iroc that I purchased 7 months ago and I have no Idea if there were any mods done. I have looked And everything seems stock but my speedo is of anywhere from 10mph at slower speeds up to 25 at high speeds ive went to stock tire sive but it still is of Anyone have any suggestions in what it could be

ps. I have found out that the rear has been rebuilt as long as the tranny and possibly the motor

thanks