I went by the MP chart once & it was dead on. But if the speedo for instance is 12 MPH slow at say 60 MPH (shows 48 MPH) you would need 20% (12 divided by 60) less teeth on the pinion than what is in there now to speed it up 20%. For example if it has a 30 in there now you'd wanna get a 24. Jack up the car as high as you can on the dr side to minimize fluid loss & as you know clock the pinion adapter on the side of the trans to match the tooth count of your new pinion.