Golf IV / Jetta IV :: Speedometer Reading Accurate Only Till Around 45 Mph

My speedometer reads accurate only till around 45 mph. The needle begins to float around gently, and the faster I go, the more discrepancy it reads. It may hang a while, floating around 60 and the speedometer app on my phone says I'm going 74.

The speed sensors were replaced for another issue a year and a half ago. The dealership initially wanted me to put a new transmission in it for $3800 after they said the trans was slipping past 45. Then they said something instead about a valve plate (could be wrong vernacular) for $1000 instead of an entire transmission.

When I finally told them that my cruise control is completely erratic, trying to track the needle movements with upshifts and downshifts, they said it's $200 in parts for the drive gears and that they were made with plastic that can wear out.

So I said I would sit on it, since the ideas about the issue seem to vary greatly, and my father set me up to go to a transmission shop to have the car checked out. They had it for two days and changed the fluid out but we can't fix your speedometer issue... take it to the speedometer shop, but btw we think your transmission is fine.

Now, do I take it back to the dealer for the drive gears, or do I take it to another specialist? I have seen people say all it needed were the drive gears replaced and a new sensor. My odometer seems to still be working. There is no issue with my gas gauge.