looks like you have a 94, you can try this cheaply to see if it helps. I did this in my 90 and it worked well. use the search there are several threads on this from the past.
QUOTE=ECR]Use the speedo drive gear form a RR Classic, FRC9339 is the part #. It will cure your roughly 5 mpg overread issue.
Cheap and easy and no need for any custom work.
Most NAS 90s came with the tan drive gear that reads too high. The Black RR Classic gear corrects the issue with 2 more teeth.
Neat piece of information. Thanks for sharing it.