Speedometer error with overdrive

Hi everyone.  I put in a "J" Type overdrive in my 76 TR6 and I now note 
about a 15 percent error in the speedometer.  I now realize that the 
speedometers for early and late, late models were different and had 
different gearing.  I also went to 205-70 15 tires but that would only 
be a 1 percent error.  What do I change,  driving gear, driven gear or 
get an earlier speedometer.  Speedometer seems like the easiest, but how 
difficult is it to change the other 2 items.  I really like the 
overdrive, seems like the engine noise just disappears when engaged.   
Thanks in advance for any help.    John Mitchell     76TR6   BRG