Here's a general outline. Always consult a repair manual specific to your 1978 Chevy truck model for precise instructions and diagrams. These instructions are a guide and might not apply exactly to your specific vehicle:
1. Identifying the Correct Gear:
* Tire Size: Measure your tire diameter (sidewall height + rim diameter x 2).
* Axle Ratio: Find this information on a sticker usually located on the driver's side doorjamb, glove compartment, or under the hood. It's often expressed as a ratio like 3.73:1 or 4.10:1.
* Speedometer Gear Chart: You'll need to find a chart specific to your truck's transmission type (e.g., 3-speed, 4-speed, automatic) that correlates axle ratio, tire size, and the correct speedometer gear number (usually a small number like 17, 18, 20, etc.). These charts are available online or in repair manuals. Incorrect gear selection leads to inaccurate speedometer readings.
2. Accessing the Speedometer Cable:
* Locate the Speedometer Cable: This cable runs from the transmission to the speedometer.
* Disconnect the Cable: Carefully disconnect the cable from both the transmission and the speedometer. You might need a specialized tool or just your hands, depending on the connection type.
3. Replacing the Gear:
* Transmission Access: You'll likely need to partially or fully remove the transmission to access the speedometer gear. This is the most challenging part, requiring specialized tools and often a transmission jack.
* Locate the Speedometer Gear: Once you have access to the transmission, locate the gear. It's a small, usually plastic gear on the output shaft.
* Removal: Carefully remove the old gear. There might be a retaining clip or other mechanism holding it in place. A small pick or pry tool might be necessary, but be very careful not to damage the shaft or surrounding components.
* Installation: Install the new gear, ensuring it's securely seated and aligned correctly.
4. Reassembly:
* Reconnect the Cable: Carefully reconnect the speedometer cable to both the transmission and the speedometer.
* Reinstall Transmission Components: Reassemble all the components you removed to access the transmission.
* Test: After reassembly, test the speedometer to ensure it's accurate. A short test drive might be needed.
Important Considerations:
* Repair Manual: A factory repair manual for your specific 1978 Chevy truck model is essential. It will provide detailed diagrams, specifications, and torque values.
* Tools: You'll need a variety of tools, including wrenches, sockets, screwdrivers, possibly a transmission jack, and possibly specialized tools to remove the gear and access the transmission.
* Safety: Working on a vehicle can be dangerous. Use jack stands properly, wear safety glasses, and follow safe working practices.
If you're not comfortable performing this repair yourself, it's highly recommended to take your truck to a qualified mechanic. Improper repair can lead to serious damage to your transmission or other vehicle components. The complexity of accessing the transmission gear varies significantly depending on your truck's specific transmission type.
