1. Check the Door Jamb Sticker (Most Reliable): The easiest method is to look at the sticker on the driver's side doorjamb. This sticker often lists the axle ratio along with other vehicle specifications. Look for something like "Axle Ratio:" followed by a number like 3.73, 4.10, etc.
2. Check the Build Sheet (If Available): If you have the original window sticker or build sheet from when the truck was purchased, that should list the axle ratio.
3. Look at the Casting Number on the Differential: This requires getting under the truck. The differential housing will have a casting number. You can then cross-reference this number (usually found on the top or side of the housing) with a Ford parts catalog or online database to find the gear ratio. This is less straightforward as you'll need the casting number and a reliable lookup source.
4. Jack Up the Truck and Measure (Least Reliable): This method is the least accurate and requires some calculation.
* Jack up one rear wheel.
* Mark the driveshaft and the tire at a common point.
* Rotate the tire exactly one full revolution.
* Count how many times the driveshaft rotates.
* The ratio of driveshaft rotations to tire rotations is your gear ratio. (e.g., if the driveshaft rotates 3.73 times for every tire rotation, your gear ratio is approximately 3.73). This is prone to error.
Recommendation: Start with the door jamb sticker. If that's unavailable, attempt to find the build sheet. The differential casting number approach requires more effort and access to information, and the jack-and-measure method is not very precise.
