Speedometer Problem

[NCBowtie]

Here's the problem. I have a 72 C10. My speedometer is way off. I'm starting to think the speedometer is bad, but perhaps there is something I am not thinking of so I figured I would see if anyone could help.

Ever since I bought the truck, the speedometer did not work at all. Transmission is TH350. The PO had bought a replacement cable so I hooked it up but still nothing. Then I hooked a drill up to the cable and confirmed that the speedometer needle would move so I assumed the speedometer was fine.

I bought a new drive and driven gear and took off the housing. There was no drive gear at all, so I figured that was my only problem. Hooked it up and now the speedometer seems to be close till about 20mph. After that it gets worse. By the time I am going about 50 it is reading 35.

I have an 8 tooth drive gear. 17 tooth driven gear. 3.07 rear and approx 29" diameter tires. According to my calculations, the speedometer should read much closer than it is. Any help would be greatly appreciated.