Speedometer Gear Question

[Kilian]

Not sure if this is the correct forum for this question, but I've got a question on what speedometer gear to use with the TH400 tranny in my 83 GMC C2500 after switching to a 4.10:1 differential ratio.

I've attached a picture of what is on the transmission now for speedometer setup. What do I need to swap to get the speedo to read correct for the 4.10:1 diff? I think the original diff was 3.73:1, as the speedo now reads higher than actual speed. Thanks for the help.

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[Craig 85]

On my 1979 K15 I had done a 3/4 ton axle swap, but ran the same height tires. I went from 3.08 to 4.10 gears. I was able to just remove that adapter and my speedo was correct. I'm probably going have to modify my adapter on my '85 when I install the new tires.

I was going to call Bob's Speedometer and see if they can help when the time comes. I've never used them though. It says on their website they can make adapter if needed. I know you can also change the plastic gear inside the transmission too. I'm thinking you need one with less teeth to slow it down.

https://www.bobsspeedometer.com/1/120/ratio_adapter.asp

EDIT: after posting this I found this calculator from TCI.

http://www.tciauto.com/tc/speedometer-gear-calculator/