Understanding your Ram truck’s specifications is crucial for various reasons, from ensuring proper maintenance to optimizing its performance for towing and hauling. One key specification is the rear axle ratio, and the 3.92 ratio is a popular choice that provides a good balance of towing capacity and fuel economy. But how can you definitively determine if your Ram truck is equipped with this particular axle ratio? This comprehensive guide will walk you through several methods you can use to find out for sure.
Understanding Rear Axle Ratios and Their Importance
Before diving into the methods, let’s briefly discuss what a rear axle ratio is and why it matters. The rear axle ratio, often referred to as the gear ratio, represents the number of times the driveshaft rotates for every one revolution of the wheels. A 3.92 ratio means the driveshaft spins 3.92 times for each wheel revolution.
A numerically higher axle ratio, like 3.92 compared to, say, 3.21, generally provides better acceleration and towing capacity. This is because it multiplies the engine’s torque. However, it can also result in slightly lower fuel economy at higher speeds because the engine has to work harder to maintain the same speed.
Knowing your truck’s axle ratio is essential for:
- Proper Maintenance: Some maintenance tasks, such as differential fluid changes, may have different requirements based on the axle ratio.
- Towing Capacity: The axle ratio is a significant factor in determining your truck’s maximum towing capacity.
- Performance Upgrades: If you’re considering performance upgrades, understanding your axle ratio is crucial for choosing the right modifications.
- Fuel Economy Expectations: Knowing your axle ratio helps you set realistic expectations for fuel economy, especially under different driving conditions.
Method 1: Checking the Axle Code on the Build Sheet or Door Sticker
The most reliable way to determine your Ram truck’s rear axle ratio is by checking the axle code, which can be found on either the build sheet or the door sticker (specifically, the Vehicle Certification Label located on the driver’s side door jamb). This method provides a direct and accurate answer.
Finding the Build Sheet
The build sheet contains detailed information about how your Ram truck was configured at the factory. You can obtain the build sheet in several ways:
- Contacting a Ram Dealership: The easiest way is often to contact a local Ram dealership. Provide them with your truck’s Vehicle Identification Number (VIN), and they should be able to retrieve the build sheet for you.
- Online VIN Lookup Tools: Several websites offer VIN lookup services that can provide you with a digital copy of your build sheet. Some of these services are free, while others may charge a small fee. Be sure to use a reputable VIN lookup tool.
- Mopar Owner Connect: If you have a Mopar Owner Connect account, you may be able to access your truck’s build sheet through their website.
Once you have the build sheet, look for a section labeled “Axle Ratio” or something similar. It will typically list the specific ratio as a numerical value (e.g., 3.92) or with a corresponding axle code.
Deciphering the Door Sticker (Vehicle Certification Label)
The Vehicle Certification Label on the driver’s side door jamb contains essential information about your Ram truck, including the axle code. The code is usually located in the bottom right corner of the sticker, often labeled “AXLE” or something similar.
Here’s how to decipher the axle code:
- Locate the “AXLE” Code: Find the section on the sticker that is labeled “AXLE.” This section will contain a two or three-character code.
- Consult a Ram Axle Code Chart: You’ll need to consult a Ram axle code chart to translate the code into the actual axle ratio. These charts are readily available online with a quick search like “Ram axle code chart.”
Here are a few common axle codes and their corresponding ratios:
- DME: 3.92 Ratio
- DML: 3.21 Ratio
- DMF: 3.55 Ratio
Important Note: Axle codes can vary slightly depending on the model year and specific Ram truck configuration. Always consult a reliable axle code chart specific to your truck’s year.
Method 2: Physically Inspecting the Axle Tag
Another method, although potentially less convenient, involves physically inspecting the axle itself for a tag that indicates the axle ratio. This method requires you to get under the truck and locate the rear differential.
Locating the Axle Tag
- Safely Secure Your Truck: Before getting under your truck, ensure it is parked on a level surface, the parking brake is engaged, and the wheels are properly chocked. Safety is paramount.
- Locate the Rear Differential: The rear differential is the pumpkin-shaped housing in the center of the rear axle.
- Search for a Metal Tag: Look for a small metal tag attached to the differential housing with one of the bolts. This tag usually contains information about the axle, including the gear ratio.
Reading the Axle Tag
The axle tag will typically have a numerical value stamped on it that represents the axle ratio. The format may vary slightly, but it will generally be clear. For example, you might see “3.92” or “3 92.” In some cases, the tag might include additional information, such as the manufacturer or the part number.
Important Considerations:
- Dirt and Corrosion: The axle tag may be covered in dirt, grease, or corrosion, making it difficult to read. You may need to clean the tag carefully with a wire brush or degreaser to reveal the information.
- Missing or Damaged Tag: Unfortunately, the axle tag can sometimes be missing or damaged, making it impossible to determine the axle ratio using this method.
Method 3: Calculating the Axle Ratio Manually (Least Recommended)
This method involves manually calculating the axle ratio by counting the revolutions of the driveshaft and the wheels. While possible, this is the least recommended method due to its potential for inaccuracy and the time it requires. It’s best used as a last resort or to confirm results obtained through other methods.
Procedure for Manual Calculation
- Safely Elevate One Rear Wheel: Safely lift one of the rear wheels off the ground using a jack and secure it with jack stands. Ensure the other rear wheel remains on the ground and is properly chocked. Never work under a vehicle supported only by a jack.
- Mark the Driveshaft and Wheel: Place a visible mark on both the driveshaft and the elevated rear wheel. This will help you count the revolutions accurately.
- Rotate the Wheel and Count Driveshaft Revolutions: Slowly rotate the elevated rear wheel exactly one full revolution (360 degrees). While rotating the wheel, carefully count the number of times the driveshaft rotates.
- Calculate the Axle Ratio: The number of driveshaft revolutions you counted represents the axle ratio. For example, if the driveshaft rotates approximately 3.92 times for each revolution of the wheel, then your axle ratio is likely 3.92.
Sources of Inaccuracy:
- Open Differential: This method only works accurately if your truck has a locking or limited-slip differential. If your truck has an open differential, the wheel on the ground will remain stationary, and the elevated wheel will spin twice as fast, giving you an incorrect reading.
- Counting Errors: It can be challenging to count the driveshaft revolutions precisely, especially if it’s rotating quickly. Even a small error in counting can significantly affect the accuracy of the calculated axle ratio.
- Tire Size Variations: Variations in tire size can also introduce slight inaccuracies in the calculation.
Method 4: Using a Diagnostic Tool (If Available)
Some advanced diagnostic tools and scan tools can read the axle ratio directly from the truck’s computer system. This method requires access to a compatible diagnostic tool and the knowledge of how to use it.
Using a Scan Tool
- Connect the Scan Tool: Plug the diagnostic tool into the OBD-II port, which is typically located under the dashboard on the driver’s side.
- Access Vehicle Information: Follow the scan tool’s instructions to access vehicle information, such as the VIN and other system data.
- Look for Axle Ratio Data: Navigate through the scan tool’s menus to find the section that displays axle ratio information. The specific location of this information may vary depending on the scan tool and the vehicle’s computer system.
Limitations:
- Scan Tool Compatibility: Not all scan tools can read the axle ratio data. You’ll need a scan tool that is specifically designed to work with Ram trucks and has the capability to access this information.
- Technical Knowledge: Using a diagnostic tool requires some technical knowledge and familiarity with the tool’s operation.
Confirming Your Findings and Seeking Expert Advice
Once you’ve used one or more of these methods to determine your Ram truck’s rear axle ratio, it’s always a good idea to confirm your findings. Cross-referencing the results from different methods can help ensure accuracy. For example, if you find a specific axle code on the door sticker, you can verify that it corresponds to a 3.92 ratio using an online axle code chart.
If you’re still unsure about your truck’s axle ratio, or if you encounter conflicting information, it’s best to seek advice from a qualified mechanic or a Ram dealership. They can use their expertise and resources to accurately determine the axle ratio and answer any questions you may have. Providing them with your VIN will be particularly helpful.
Determining your Ram truck’s rear axle ratio is essential for various reasons, from ensuring proper maintenance to maximizing its performance. By using the methods described in this guide, you can confidently identify whether your truck has a 3.92 rear axle ratio and make informed decisions about its care and operation.
How can I find the build sheet for my Ram truck to determine the axle ratio?
Your Ram truck’s build sheet is a detailed document outlining the original specifications of your vehicle, including the rear axle ratio. There are several ways to access this information. You can often retrieve it online by entering your VIN (Vehicle Identification Number) into the Mopar Owner Connect website. Alternatively, you can contact your local Ram dealership with your VIN and request a copy of the build sheet.
The build sheet will list all the factory-installed options and features of your truck, including the rear axle ratio. Look for a section detailing the axle specifications. The code for a 3.92 rear axle ratio will typically be clearly indicated, allowing you to confirm whether your truck was originally equipped with it. Keep in mind that the build sheet reflects the original factory configuration.
Where is the axle code located on my Ram truck, and how do I interpret it?
The axle code on your Ram truck is usually found on a metal tag affixed to the axle housing itself. It can sometimes be difficult to locate due to dirt and grime accumulation, so you might need to clean the axle housing to find it. The tag will contain a series of numbers and letters that represent the specific axle type and ratio.
Once you locate the tag, you will need to find the specific code related to the axle ratio. The code for a 3.92 ratio might vary slightly depending on the year and model of your Ram. Consult a Ram truck resource, such as a service manual or online forum, to interpret the code accurately. This will confirm if the axle currently installed is indeed a 3.92 ratio.
Can I use the Vehicle Identification Number (VIN) to determine my Ram truck’s rear axle ratio?
Yes, the Vehicle Identification Number (VIN) is a valuable tool for determining your Ram truck’s rear axle ratio. Many online VIN decoders and vehicle history report providers can access the original factory specifications using your VIN. These reports often include information about the rear axle ratio.
While a VIN decoder provides valuable information, it’s crucial to remember it reflects the original factory configuration. If the axle has been replaced or modified, the VIN information will not reflect the current axle ratio. Always cross-reference this information with other methods for confirmation.
What is the easiest way to visually identify a 3.92 rear axle ratio on my Ram truck without any tools?
Unfortunately, visually identifying a 3.92 rear axle ratio without any tools or specific knowledge of the axle components is difficult. There are no external markings or visual cues that definitively indicate the gear ratio. The differential housing looks similar regardless of the ratio.
The most reliable methods involve checking the build sheet, axle code, or performing a manual rotation test. While you might be tempted to rely on visual inspections, remember that those are not reliable. Accurate determination requires consulting documentation or performing a physical test as explained later.
How can I perform a manual rotation test to determine the rear axle ratio of my Ram truck?
A manual rotation test involves lifting one rear wheel of your Ram truck off the ground (make sure to use jack stands for safety). Mark the tire with a piece of tape at the top. Then, with the truck in neutral, rotate the driveshaft by hand and count how many full rotations of the driveshaft are required to make the tire complete one full rotation.
The number of driveshaft rotations will be close to the axle ratio. For a 3.92 ratio, you should see approximately 3.92 rotations of the driveshaft for one tire rotation. This test provides an estimate, and it’s best to compare the results with other methods for verification, as slight inaccuracies can occur. Remember to chock the wheels and engage the parking brake before starting this test.
Does the towing package on my Ram truck automatically mean it has a 3.92 rear axle ratio?
No, having a towing package on your Ram truck does not automatically guarantee that it has a 3.92 rear axle ratio. While towing packages often include features that complement a lower (numerically higher) axle ratio, the specific ratio included can vary depending on the model year, engine configuration, and other options selected.
The towing package generally enhances the truck’s towing capabilities, but the rear axle ratio is a separate component. You must verify the actual axle ratio using the methods previously mentioned, such as checking the build sheet or axle code, rather than assuming it based on the presence of a towing package.
Will changing tire sizes affect my Ram truck’s performance if it has a 3.92 rear axle ratio?
Yes, changing tire sizes can significantly affect your Ram truck’s performance, regardless of whether it has a 3.92 rear axle ratio or not. Larger tires effectively change the overall gear ratio, which can impact acceleration, fuel economy, and speedometer accuracy.
With a 3.92 ratio, which is already geared towards better acceleration and towing, installing significantly larger tires can make the truck feel sluggish, particularly at lower speeds. Conversely, smaller tires can improve acceleration but reduce fuel economy at higher speeds. Re-calibrating the speedometer and considering the impact on transmission shift points are essential when changing tire sizes.