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That is correct bmorgil, a mis-matched set will end up in disaster trust me I learned the disaster way :(:(:(

Try this website, it may help

https://www.tciauto.com/speedometer-gear-calculator

Here is another site that has speedo adaptors that I have used for my speedo

https://www.texasindustrialelectric.com/speedo.asp

Something has me a little puzzled.
A few threads back David you mentioned it was a “single shifter Dana 18 transfer case”
“Doctor Dana” would be able to answer this, what year did they switch from...