Any time you change gearing in the car, tire size, tranny, rear end, etc, the speedometer and odometer will be moved out of spec by the same amount. That is true.
However, over time the spring can weaken, causing your speedometer to read a higher number than you are actually going, while the odometer will still read the correct amount, that is because the odometer is directly geared to the speedometer cable, so any time the speedometer cable moves 1026 revs, the odometer has moved 1 mile, that's just the way it is and there's nothing that can change it. So, what happens when your odometer reads correctly, but your speedometer does not? You need to recalibrate the speedometer to read correctly.
That's where the calibration of the speedometer comes in. You can adjust it both higher and lower, and that calibration adjusment can move actually pretty easily. People who get speedometers and clusters shipped, shipping could easily move that adjusment slider.
The whole odometer/speedometer issue really is that you need to make sure the odometer is reading correctly before making adjustments to the speedometer, because if the odometer isn't reading correctly, you have a gearing problem that should be fixed before anything else. Once you have the odometer reading correctly, speedometer calibration can be done if your speedometer still is not reading correctly.
I hope that explained things a bit better, haha.
