* Decal on the Differential: Look for a sticker on the differential housing itself. Sometimes, the gear ratio is printed there. This is the easiest method if the sticker is still present and legible.
* Jacking Up and Rotating the Tires: This requires some basic tools and mechanical aptitude.
1. Secure the vehicle: Jack up the rear end of the Astro securely, using jack stands. Safety first! Never work under a vehicle supported only by a jack.
2. Chock the front wheels: Ensure the front wheels are chocked to prevent the vehicle from rolling.
3. Mark a tire and the driveshaft: Make a mark on one tire and on the driveshaft (easily visible).
4. Rotate the tire: Rotate the tire exactly one full revolution.
5. Count driveshaft rotations: Count how many times the driveshaft rotated.
6. Calculate gear ratio:
* If the driveshaft rotated 3.73 times for one tire rotation, your gear ratio is 3.73:1.
* If the driveshaft rotated 4.10 times, your gear ratio is 4.10:1. And so on. Common ratios for these vehicles are 3.42, 3.73, and 4.10.
* Vehicle Identification Number (VIN): The gear ratio might (but not always) be encoded in the VIN. You'll need to use a VIN decoder service or consult a parts database specific to GM vehicles (e.g., using the VIN on a parts website). This method is not guaranteed to provide the information.
* Check the Build Sheet (if available): If you have the original window sticker or build sheet for the vehicle, it might specify the rear axle ratio.
The jacking-up method is the most reliable if the other options fail, but it demands careful attention to safety. Always prioritize safety when working under a vehicle. If you're not comfortable performing this procedure, it's best to take your Astro to a mechanic.
