I have a 91 mustang gt the car has 4.10s the speedo is off to much and is very anoying. I called over at 50resto and they said I need the 7 tooth, but want to make sure. I was searching on this site and seems others with the same trans 1990 and up are using the 6 tooth. Anyone positive on this, I will be ordering this week both drive gear and speedo gear