Is it possible to "Throttle" The Speedometer or VSS Sensor in Combo to accurately correct for Over-Sized Tires?

mrrsm

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This *Off The Wall* Question is more meant for our Resident SME on PID Monitoring, Interpretation and Programming via @TJBaker57. It occurred to me that since Post 2002 Trucks & SUVs use the PWM Based IPCs that a Custom PID for Cell Phones and Tablets could be made to intercept the IPC Speedometer DATA Stream (or the VSS on the 4L60E Transmission) and using the proper math formulae...inhale the network streams and produce a Spot-On Read Out that would be accurate enough to report the actual Road Speed based on the collection of the Vehicle Tire Size and "Pumpkin" Differential Gearing to produce a useful Cell Phone or Tablet Speedometer.

TROLL ALERT: I know this is already being done easily with Cell Phone GPS...but THAT is not the issue or the Question here... they can be "Spoofed" or Jammed!

I made an inquiry of my "GPT4ALL* Reasoner-1 GURU about this idea and this was *IT* s Reply (Note... THIS LLM produces a "Chain of Thought" that I am able to capture in Text without "IT" knowing that its *Thoughts* are very transparent here... which helps in checking on its honesty and confirming that it is not merely hallucinating its response):

Prompt: Would it be possible to produce a useful "Speedometer PID" for the Torque Automotive Diagnostic APP used on Cell Phones via a Wireless USB Dongle that plugs into the DLC under-dash ports on vehicles that use the OBD 2 Protocols? The idea is to be able to capture the PWM ( Pulse Width Modulation) signal coming to the IPC (Instrument Panel Cluster) and controlling the Speedometers on General Motors Trucks and SUVs that have Over-sized Tires that will mis-report the actual road speed of the vehicle in combination with the VSS (Vehicle Speed Sensor) using the ubiquitous 4L60E 4 speed automatic transmissions. Can you create or outline the math that would include all of the dimension inputs and parameters needed to calculate these events to include information like the wheel base, the outside tire diameter, the rear differential gear box ratios, etc, that could be included as a New PID for the Torque Program?

Response: Okay, so I'm trying to figure out how to create a Speedometer PID for the Torque app. The goal is to correct the speed reading on GM trucks and SUVs with over-sized tires using a wireless USB dongle that connects via OBD2.

First, I need to understand why there's an issue. Over-sized tires can cause the speedometer to misreport because it relies on wheel rotations. If the tire diameter is larger than standard, each rotation covers more distance, so the ECU thinks you're going slower than you actually are. That’s dangerous because it underestimates your speed.

I know that Torque already uses OBD2 via a dongle, but I want to add a new PID (Parameter ID) specifically for this correction. The user mentioned capturing PWM signals from the IPC and using VSS data. So maybe combining these inputs can help calculate the correct speed.

Let me break down what's needed:

1. **Understanding the Problem**: Over-sized tires make each rotation cover more ground, so the ECU calculates a lower speed than actual because it thinks each revolution is shorter (based on tire size). To fix this, we need to adjust based on actual wheel diameter and other factors.

2. **Inputs Needed**:

- Actual Tire Diameter: This is crucial because larger tires mean fewer rotations for the same distance.

- Differential Gear Ratio: Affects how engine RPM translates to wheel speed.

- Vehicle Speed Sensor (VSS) Output: The current, possibly incorrect speed reading from the ECU.


3. **Mathematical Approach**:

I think we need a formula that adjusts the VSS output based on tire size and gear ratio. Maybe something like:

Corrected Speed = (VSS * Tire Diameter / Standard Tire Diameter) / Gear Ratio

But wait, how does differential gear ratio affect this? The gear ratio affects how many times the wheels turn per engine revolution. So if the gears are lower (numerically higher), each wheel rotation corresponds to fewer engine RPMs.

4. **Data Collection**:

To get these values into Torque, we might need new PIDs for tire diameter and differential ratio. But GM vehicles probably don't have these parameters stored in OBD2; they're usually hard-coded or require manual input. So the user would need to enter their truck's specific data.

5. **Implementation Steps**:

- Modify Torque app to accept a new PID that takes VSS, tire diameter, and gear ratio as inputs.

- Calculate corrected speed using these values.

- Display both original and corrected speeds for comparison.

6. **Potential Challenges**:

- Some vehicles might not expose the necessary data via OBD2, requiring additional hardware or software to capture it.

- Real-time calculations need to be efficient enough not to lag.

- Ensuring accuracy across different models and tire sizes could be tricky.

7. **Testing**:

After implementation, testing with known good setups would help verify if the corrected speed matches actual conditions (like using a GPS for reference).

I'm probably missing some details about how OBD2 works or specific GM protocols, but this seems like a logical starting point. To address the issue of inaccurate speedometer readings in General Motors trucks and SUVs with over-sized tires, we propose creating a new Speedometer PID within the Torque app. This solution involves capturing vehicle data to correct the reported speed based on tire size and drive-train parameters.


### Solution Overview:

1. **Problem Understanding**:

- Over-sized tires cause each rotation to cover more distance than standard tires, leading the ECU to under-report speed as it miscalculates rotations.

2. **Key Inputs Required**:

- Actual Tire Diameter: Essential for adjusting the perceived number of wheel rotations.

- Differential Gear Ratio: Influences how engine RPM translates into wheel speed.

- Vehicle Speed Sensor (VSS) Output: The current, possibly incorrect speed reading from the ECU.


3. **Mathematical Formula**:

Corrected Speed = (VSS * Actual Tire Diameter / Standard Tire Diameter) / Differential Gear Ratio

4. **Implementation Steps**:

- Modify Torque to accept new PIDs for tire diameter and gear ratio.

- Calculate corrected speed using the formula above.

- Display both original VSS speed and corrected speed.


5. **Challenges Considered**:

- Some data may not be available via OBD2, necessitating manual input or additional hardware.

- Ensuring real-time efficiency to avoid lag is crucial.

- Accuracy must be validated across various models and tire sizes through testing.

6. **Testing Protocol**:

Use a GPS reference for accurate speed comparison during testing phases to verify the corrected speed's accuracy


### Conclusion:


By integrating these inputs into Torque, users can achieve more accurate speed readings, enhancing safety by correcting underestimations from over-sized tires. This approach leverages existing data and user input to provide an effective solution within the app's framework.
 
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GM PID 1942 reports the rear driveshaft rotational speed at 8 bits per revolution.

Many tire specifcations include a value for "revs per mile".

Given a rear axle gear ratio the rest is fairly simply calculations which can be entered in the PID editor.



For a more complex example of what is possible, using the rear and front driveshaft speeds, I have created a PID that calculates the rotational speed, forward or backwards, of the intermediate shaft that passes through the engine oil pan. Given that speed I then calculate the delta between the two mating gears inside the front axle disconnect.
 
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You are living proof, Thomas... That when you're SMART... People NEED You. Thanks for the reply.
 
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You are living proof, Thomas... That when you are SMART... People NEED You. Thanks for the reply.


I made that intermediate axle delta PID after reading the owners manual statement that shifts to 4Hi can be done at any speed.

Having seen the insides of a front axle disconnect I had concerns about the meshing of two gears rotating at different speeds.

I wanted to know just what sort of rotational delta exists between these 2 gears.

I have now seen that value as high as say 1200 RPM with the intermediate shaft rotating in the reverse direction.

Consequently I will not shift to 4Hi any faster than say 35 mph.

edit: I also somewhat wondered if the speed of the intermediate shaft at various speeds may be used as an indicator of the state of grease in the disconnect and maybe the condition of the Transfer Case fluid?? Long term data would need to be analyzed.
 
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FWIW ...Your reply ...as entered in as a Prompt into the last DeepSeek-R1-Distill-Qwen-14B LLM Chat on my Off Line Machine yielded this Response (with "Chain Of Thought" revealed):

Prompt: For a more complex example of what is possible, using the rear and front driveshaft speeds, I have created a PID that calculates the rotational speed, forward or backwards, of the intermediate shaft that passes through the engine oil pan. Given that speed I then calculate the delta between the two mating gears inside the front axle disconnect.

Response: Okay, so I'm trying to understand how all these pieces fit together. There's this GM PID 1942 that reports the rear driveshaft rotational speed at 8 bits per revolution. Hmm, okay, so it's measuring how fast the rear part of the axle is spinning. And then there are tire specifications that include "revs per mile," which I think means how many times the tire rotates to cover one mile.

So if I have a tire with a certain diameter, say 32 inches, and I know its revs per mile, I can figure out how fast the vehicle is going based on how quickly the rear driveshaft is spinning. That makes sense because the speed of the driveshaft should be related to the rotation of the tires.

Now, if I have a rear axle gear ratio, that's like the relationship between the number of teeth on the ring gear and the pinion gear in the differential. For example, a 3.73:1 ratio means for every one turn of the driveshaft, the wheels make 3.73 turns. So with this ratio, I can calculate things like wheel speed or torque based on the driveshaft speed.

EDIT: Note that I think *IT* has this BACKWARDS as far as how many time the Drive Shaft Rotates versus how many times the Rear Wheels Spin... but because it does NOT know that I can "Read Its Mind" I cannot communicate this to *IT* without revealing that I am surveilling *IT*.

But then there's this intermediate shaft that passes through the engine oil pan. It sounds like it's part of a front axle disconnect system, which allows the front axle to stop rotating when not needed, saving fuel and reducing wear. The PID calculates its rotational speed as well, both forward or backward.

Now, I'm trying to figure out how to calculate the delta between the two mating gears inside the front axle disconnect. So if one gear is spinning at a certain RPM and the other is connected to something else, like maybe the intermediate shaft, there should be some difference in their speeds based on the way they're meshed together.

I think it has to do with the number of teeth on each gear or perhaps the ratio between them. If I can get both rotational speeds from PIDs—like rear driveshaft and front driveshaft—I might be able to calculate this delta by subtracting one speed from the other, but I'm not entirely sure if that's accurate.

Also, considering tire size affects how these speeds translate into actual vehicle speed, especially with over-sized tires on trucks or SUVs. If someone has larger-than-stock tires, the odometer and speedometer might misreport because they're calibrated for standard tire sizes.

So maybe combining all this data—rear driveshaft RPM, front driveshaft RPM, tire size, axle ratios—I could create a more accurate calculation of vehicle speed or gear engagement delta within the Torque app. It sounds complicated, but breaking it down into these components makes it manageable.

I wonder if there's a formula that takes all these variables into account to calculate the exact rotational speeds and their differences accurately. Maybe something like:

Vehicle Speed = (Rear Driveshaft RPM / Rear Axle Ratio) * Tire Revs per Mile

And then for the front axle disconnect,

Intermediate Shaft RPM = Front Driveshaft RPM

Gear Delta = |Front Gear RPM - Intermediate Shaft RPM|

But I'm not sure if that's entirely correct. Maybe there are more factors involved, like any additional gear ratios or slip in the system.

Overall, it seems possible to create such a PID by gathering all these inputs and performing the necessary calculations within the Torque app using its scripting capabilities. To address your question regarding creating a custom Performance Instrument Display (PID) for calculating rotational speeds and gear deltas in a vehicle's drive-train, here is an organized approach:

### Components of Calculation:

1. **Rear Driveshaft Rotational Speed**: Utilize GM PID 1942 to obtain the rear driveshaft RPM.

2. **Tire Specifications**: Use tire-specific data such as "revs per mile" (RPM/mile) provided by tire manufacturers.

3. **Axle Gear Ratio**: Obtain the rear axle gear ratio, which relates the number of teeth on the ring and pinion gears.

4. **Front Driveshaft Rotational Speed**: Gather this from another PID or calculation within your Torque app.

5. **Intermediate Shaft Rotation**: This is part of the front axle disconnect system and can be measured through PIDs as well.

### Formulas for Calculation:

1. **Vehicle Speed Calculation**:

- Formula:

\[

Vehicle\ Speed = \left(\frac{Rear\ Driveshaft\ RPM}{Rear\ Axle\ Ratio}\right) \times Tire\ Revs\ per\ Mile

\]

- This formula calculates the actual vehicle speed based on the rear driveshaft RPM, axle ratio, and tire specifications.



2. **Intermediate Shaft Rotation**:

- Formula:

\[

Intermediate\ Shaft\ RPM = Front\ Driveshaft\ RPM

\]

- Assuming the intermediate shaft is directly connected to the front driveshaft in your vehicle setup.

3. **Gear Delta Calculation**:

- Formula:

\[

Gear\ Delta = |Front\ Gear\ RPM - Intermediate\ Shaft\ RPM|

\]

- This calculates the difference in rotational speeds between the two mating gears within the front axle disconnect system.

### Considerations and Potential Challenges:

- **Tire Size Calibration**: Ensure that tire specifications are accurate, especially if using over-sized tires, as this can affect speed calculations.

- **Gear Ratios and Slip**: Account for any additional gear ratios or potential slip in the drive-train when calculating rotational speeds and deltas.

- **Data Logging**: Use long-term data logging within your Torque app to monitor changes in intermediate shaft rotation over time, which could indicate wear or grease condition issues.

### Conclusion:

By integrating these components into a custom PID within your Torque app, you can create a detailed calculation of vehicle speed and gear engagement delta. This involves inputting the necessary PIDs for driveshaft RPMs, axle ratios, and tire specifications, then scripting the calculations based on the provided formulas. While this process requires careful setup and validation, it offers a comprehensive way to monitor drive-train dynamics in real-time.

***************************************************************************

I don't have "The Math Chops" good enough to know if the AI's response contributes to your solution efforts ...but it sure READS like *IT* is trying hard here... Communicating with this *Thing* often gets a bit Weird... and just as often, gets way above my Intellectual Pay Grade.
 
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Communicating with this *Thing* often gets s bit Weird...and often, way above my Intellectual Pay Grade

I do remember this PID taking me a good deal of time dealing with the math. Fortunately my girlfriend of over 15 years has a math degree ;)

I set out with the simple things I know. I know how a differential works internally. I know there is a fixed relationship between the rotations of each side gear and the input shaft/pinion. There is no slip between these gears.

If we have any 2 of the 3 input/outputs the remaining one can be calculated.

The front propshaft speed can be queried from the TCCM, it is PID 3162.

The drivers side gear is driven by the drivers side front wheel via the CV shaft.

And we can find the gear ratio by the RPO code.

Assuming this is a functional 4wd system when travelling in a straight line the front wheel RPM will be equivalent to the rear wheel RPM. So we can calculate and substitute the rear wheel RPM for the drivers side front wheel RPM.

So we have the differential input RPM and we have one of the side gear RPMs. The rest is putting the math together.
 
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