Sometimes, it really is better to be lucky than good. I tried finding information on the internet about connecting a GM style speedo to a Toyota transmission, or a Japanese transmission of any kind. I couldn't find any info at all, so I figured I would find out the hard way how to make it work, and if the drive ratio was anywhere close.
GM Speedos use a 5/8"-11 thread on the back with a 1/16" square drive, like the Speedway gauge set I am using:
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The Toyota trans uses an M18 thread with a round drive and a crimped 'paddle' drive, which is common among other Japanese transmissions, including early Miatas:
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The speedo cable for the 1979 Toyota Pickup donor utilized a square drive on the speedo end, but has a simple clip hook that hooked into a groove on the back of the speedo:
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It turns out, that this hook setup will snap on to the 5/8"-11 thread of a GM speedo and the square drive will spin the speedo!!!
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The icing on the cake: the speedo reading is within ~2mph of a cell-phone GPS speed reading!!! That's as good as a factory car!!!!
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