Doug - Yes, the actual ring and pinion tooth count does help! If your 1954 3100 has a 39 tooth ring gear, the corresponding pinion gear should be a 10 tooth gear. Equals a 3.90 rear axle ratio. That combined with your known SM420 4 speed and 16” wheels should give us the clues needed to track down the correct speedo gear set.
More on this in the morning when brain cells and eyes are fresher.
