What it does is ratio the signal from the speed sensor to the PCM; that one looks like it plugs in right at the speed sensor connector. This is the kind of device I mentioned above. There are several dip switches inside the unit; you set them in a particular order to match the correction factor you need to make the speedo read properly (a list is included with the device). If you currently have 3.27 gears and are installing 3.73s, your speedo will read fast, so divide 3.27 by 3.73 to come up with a .876 correction factor, set the switches to most closely match .876 and you'll be real close.
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