speedometer gear question

just pulled the gear and it is a 33 tooth gear. According to the chart I need between a 34-36 gear. I am 8 MPH fast according to an accurate speedo. How much MPH per tooth does it slow down. If I have a 33 now and need to slow it down 8MPH at 70, would I need a 34 tooth or should I go with a 35. Thanks