so i was going 80 through town yesterday...

You know how a Speedo works?
It is a close tolerance magnetic coupling driven by the cable.
The cable wouldn't rotate faster if damaged.
Normally trash gets in the gap causing the dreaded flopping needle.
Or the shaft is worn causing the rotating parts to collide.
Not a cable. It either turns or it doesn't. I'm just saying cables aren't my normal problem.
I’ve never heard of your problem.
But since this is a cable drive and you are sure you are not hydroplaning and unless there is voodoo in the transmission gear that is the only thing I can figure.
In other words something is pulling the needle more than it is supposed to.
And it is being dampened too, which prevents it from flopping.
This is a WAG on my part.