Need help from a speedometer gear size expert!

Hi guys. I have a 225 powered 74 Duster with the TF A-904 auto tranny. The tranny was rebuilt last year and I don't know the tooth size of the speedometer drive gear that they put in. My completely stock speedometer reads four mph slow. If I change this gear, will my speedo read correctly? I think I need fewer teeth. How many teeth reduction would it take to make my speedometer speed up by 4 mph? Any ideas? Thanks.