Phil,
As I understand it, the speedometer drive gear in the transmission should not need to be changed. It is the gear on the end of the speedometer cable that gets changed depending on rear axle ratio and tire size. According to the Plymouth parts book, your P10 with a 4.1:1 rear axle and 6.00X16 tires should have a speedometer gear with 18 teeth. Do you know what you have in there now? It might be worth it to pull out the end of the speedometer cable from the transmission and count the number of teeth. It sounds like it may have a gear with less than 18 teeth making the speedometer read faster than actual speed.
Jim Yergin