1. The VIN (Vehicle Identification Number): While not always reliable for the rear axle ratio, the VIN *might* contain a code that gives you a clue. You'll need a Chevrolet VIN decoder specific to the 1978 model year. These decoders can be found online. However, the VIN often doesn't specify the exact rear end ratio, only the axle type (e.g., 10-bolt, 12-bolt) which narrows it down but doesn't provide the numerical ratio (e.g., 3.73, 4.10).
2. The Build Sheet (if you have it): A build sheet from the factory will specify the rear axle ratio.
3. The Differential Tag (most reliable): Look for a tag on the differential housing itself. This tag will usually list the gear ratio. The differential is located in the rear axle assembly under the truck. You might need to clean the area to see it properly.
4. Manual Calculation (requires some work):
* Jack up the rear end: Securely lift one rear wheel off the ground using jack stands.
* Mark a point on the tire and the driveshaft: Use chalk or paint to mark a reference point on both the tire and the driveshaft.
* Rotate the tire exactly one full revolution: Carefully rotate the tire one complete turn.
* Count the driveshaft revolutions: Count how many times the driveshaft rotated.
* Calculate the ratio: The number of driveshaft revolutions is your rear axle ratio. For example, if the driveshaft rotated 3.73 times for one tire revolution, your ratio is 3.73:1.
Important Considerations for Method 4:
* Accuracy: This method requires precision. Even a small error in the tire rotation or driveshaft count can affect your result.
* Safety: Always use jack stands and follow safe jacking procedures. Working under a vehicle is dangerous.
Which method is best? Checking the differential tag (method 3) is the easiest and most reliable way. If that tag is missing or unreadable, the manual calculation (method 4) is a viable alternative, but be very careful and methodical in your measurements. The VIN (method 1) and build sheet (method 2) provide less certain information.
