Not true. These are 2 totally different instruments in the one house. The odometer is directly gear driven. So cable rotations, less gears jumping teeth, directly effects odometer. So correct that the speedometer drive gear effects odometer. Odometer correctness can be a good indicator of speedometer drive gear mathematics correctness. At the same time, Speedometer is not direct driven. You could duct tape the speedometer needle at zero and drive the car. Odometer would still operate without change.
And without quoting anothers post, if the magnets did get weaker, the speedometer would run slower, not faster LOL