The calibration of the speedometer is usually done in the speedometer not the cable. There is a torsion spring that can be calibrated to match your new gears and tire size. Another way might be to use a GPS system to make a mental calibration so you know how fast you are going compared to what speedometer reads. I use a GPS system exclusively as a speedometer and in the unlikely event that I should get "lost" I don't have to stop for directions.

1953 Chevrolet 3600
Fried Green T'mater (Vern)
In the Stovebolt Gallery
Caution: Political Statement
Man who runs in front of car gets tired, man who runs behind car gets exhausted.