Need help from a speedometer gear size expert!

Convert the error to percent, then change the existing gear by that percent.

For instance 4 mph at 30 mph is 13%, but at 60 it is only 6.66%
If you have a 27 tooth the former is 3.5 teeth and the latter is 1.8 teeth

If your speed-O reads high, and you need to slow it down, this will require more teeth, and vice-versa.