Did you say what year your Mustang is and what kind of speedo it has? I have an 89 1/2 Mustang since it has an 140mph speedo. Charts tell me I need to go to a 20 tooth if I have 3.55 gears, but in reality I need to go to a 23 tooth. If you have an 89 with a 140mph speedo, follow the 1990 chart and not the pre-1989 chart for gears.
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351W 89 Mustang GT Convertible