[dmsfrr]

The speedometers tended to read a bit high anyway. I guess it was a marketing ploy???
Changing the tires to a set with a slightly smaller overall diameter accentuates the problem.
Are you checking it against a known correct speedometer? Mile markers on the hiway? GPS or ?
Has the speedometer been cleaned and serviced recently? If not, it may just need attention.

Adding a tooth to the driven gear on the lower end of the speedo cable will slow the speedometer approx 5% (it's been a very long time since I did the calculations to change one) so a replacement gear with 24 teeth instead of 22 may be about right.
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