If your speedometer is inaccurate, the problem is likely one of the following:
* Faulty Speed Sensor: This is the most common cause. The speed sensor sends a signal to the speedometer based on the rotation of the driveshaft. A faulty sensor will send an incorrect signal, leading to an inaccurate reading. This needs to be replaced.
* Incorrect Tire Size: If you've changed to tires with a different diameter than the original equipment, the speedometer will be off. Larger tires will make the speedometer read slower than your actual speed, and smaller tires will make it read faster. This is not a speedometer adjustment; it's a recalibration based on the tire size. You can use an online tire size calculator to determine the percentage of difference and adjust your speed accordingly, or you can get a speedometer calibrator that accounts for this.
* Transmission or Differential Gear Ratio Issues: If the gear ratios in your transmission or differential have been changed (e.g., through gear swaps), this will also affect the speedometer's accuracy. This requires recalibration or replacement of the speedometer gear if it's a cable-driven speedometer.
* Speedometer Cable (if cable-driven): On some 1992 Silverados (depending on the trim), the speedometer may be cable driven. A broken or frayed cable can lead to inaccurate readings or a completely inoperative speedometer. This cable needs to be replaced.
* Speedometer itself: In rare cases, the speedometer mechanism itself might be faulty. This would require replacement of the gauge cluster.
In short: Instead of "adjusting" the speedometer, you need to *diagnose and repair* the underlying cause of the inaccuracy. A mechanic specializing in older vehicles can help you pinpoint the problem and perform the necessary repairs or replacements. Starting with checking the tire size and then inspecting the speed sensor is the best place to begin.
