Speedo jumps

[Blaze4g63]

Does anyone know why my speedo but jumps way up and then comes back down every time I brake my trucks a k1500 92

 

[Calvinator]

I had similar problem. It was my ignition switch. Appears a common issue with GMT400.
The other symptom was the illumintated gear indicator in the dash would flicker bright to dim to bright.

My mechanic figured it out, so I don't know his troubleshooting process.

Something to research!