Help, someone tell me what speedo gear i need!

OK, im starting to get lost in the sea of formulas and charts for speedo gears. can someone tell me what gear i need. here is my info.
1965 mustang
1970+ c4 tranny (26 spline style)
215 60r15 tires tire height is 25.? inches i believe
3.55 posi rear end 8"

I cant find searching the correct way to calculate the number of gear teeth i need. thanks for any help! And im assuming there is some difference between early c4's and later c4's (maybe driven gear teeth) because when i replaced my tranny and still had 2.79 gears, the speedo was off with the same drive gear so hummm! thats why i threw that bit of info in there.

ok, nevermind. i just realized that i have a autometer electric speedo and they are programmable. so i wont need to change the gear. all i have to do is drive a known 2 mile distance with the speedo in program mode.