are you running a T5 trans? if so then thats what you need to do. i was running the 21 tooth red gear with the stock inside gear for a while and my speedo still seemed fast, so i found out that you need to change the inside (drive gear) as well. you need the yellow 7 tooth drive gear and the red 21 tooth driven speedo gear to be as closely calibrated as possible, if you are running a stock size wheel and tire combo. good news is that you can get both of these parts through the ford dealer. i will post the part numbers if i can find them later.
oh yea the stock inside drive gear is the green 8 tooth.
oh yea the stock inside drive gear is the green 8 tooth.