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When I installed the speedo cable adapter I set it to what I thought would be correct according to the directions. I set it for the range of a 3:55 rear axle which is what we installed. Car is originally a 2:94 rear. Problem I had was the speedo wasn't working. I investigated and found that the gear was not touching the gear in the trans. When I rotated it to where it may go for the original 2:94 rear end it touches and works. This IS the original gear to the car. My question is when replacing the rear axle gear does this mean the replacement speedo gear is a different diameter? The adapter is oblong so to speak so that in different positions the gear is a different distance from the gear in the trans. Maybe the 2:94 speedo gear is smaller diameter and when I put the adapter in the 3:55 setting it will not touch? Forgive my mechanical ignorance I'm a body guy!

Yes when I ran 2:94 I had a 29 gear and a 35 for a 3:55 gear which is what I'm running now. There is a formula for tire size and ratio somewhere. I have a Cyberdyne digital speedo now so I have my wife drive at a steady 45 mph and I just push a button and it calibrates itself. Pretty handy because I have 3 sets of gears I can swap in depending on the occasion. 2:94, 3:55, 5:13s. Also the speedo gear holder is marked at different points depending on ratio. Hope this helps.

There are two different settings you are talking about. The speedo gear is determined by axle ratio and tire size. How ever, the adapter is rotated for teeth, not axle ratio. So if you have a 23 tooth gear, you rotate the adapter to fit the until the number range lines up with the mark.

Hope this helps.

Yes ws27 is correct about the adapter, it's marked for tooth count not actual ratio. I miss spoke.

That's a nice chart there mark.