I have a 1968 xr7, just installed aod trans from a c4. speedo is 10mph fast and current gear for speedo is 16 teeth. what would be the proper gear
Google is your friend. There are many speedometer gear calculators. All of the ones that work need a lot more information than you have given. Look here:
https://www.mercurycougar.net/threads/reference-information-speedo-gears.5657/