Speedometer Problem Issues

[1967KaiserM715]

So, this morning was cold, about 20 degrees. nothing to abnormal yet, drove truck to the gas station to get my morning coffee, everything was OK, but the minute I get to the highway(speed limit of 55MPH, but typical speeds are around 60MPH) I realized my speedometer was hovering right around 80MPH, and I hadn't even shifted into OD yet(6.5 Turbo-Diesel with NV4500 and NP208) typical speedometer reading with RPM reading of 2,000RPM is roughly 45MPH,

Also should say here, the Speedometer has always been a little off, I have 31" tires(that are measured at 28.5" actual diameter) and as such without changing the driving gear, my speedometer has been off by roughly +5MPH at 50MPH actual, or a reading of 55MPH

so anyways, this evening on the drive home I paid closer attention, it seems like the speedometer reads OK at speedometer readings under 35, but above 35MPH readings, it will read between 15MPH or 25MPH OVER indicated speed, and the needle is a little bouncy, actually it has been a little bouncy right at the 45MPH mark since I've had the truck, but steady otherwise until now.

any input is appreciated, Thanks in advance