Incorrect Speedometer Reading

[my67c20]

TH350 trans, 3:73 rear gear, 29.5" tires, speedometer ~10 miles fast, verified by a GPS at different speeds. I tried some of the online tools, nothing made me feel confident in what they mentioned. Not 100% sure of size of output shaft on trans, apparently there is 1.84 & 1.76. Hard to tell colors from pictures, but looks like brown on trans (drive) and natural (driven) on end of speedo. However, no charts mention the brown drive gear, so it makes it difficult to calculate. Help is appreciated and I did search, but nothing really relevant was found.