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That's pretty much what I did originally over 10 mile stretches.
But last tire upgrade, I used a GPS and compared that to the indicated speed. Did the math to find a multiplier. 68 indicated and 60 actual is 68/60 = 1.133 for example . Then checked what speedo gear was in the trans and applied the constant. The speedo gear is right there when you didconnect the speedo cable from the trans.
If your speedo is reading high, you need more teeth on the gear. So if you have a 30 tooth gear now, multiply by 1.133 to get 34.0 teeth required. Went on line and bought a new gear with the correct number of teeth (or as close as you can get)
Now the speedo matches the GPS.