Knowing that this is a modified line I did some back calculating and I believe your factory AS BUILT lines would be close to this:

726-15-01 C104 0163 09??
726-15-02 95?? ????

The 95 in the second line could be in the 93-97 range. If that is true, you could try the following lines and you should be close on your speedo.

726-15-01 C104 014B 0975
726-15-02 8C16 898F

If my guess at your factory AS BUILT lines is not correct, post the originals and I will do a recalculate.

Let us know.

 

I dont know why the truck would ever consider the gear ratio when reporting speed. It has 4 ABS sensors at the hubs to measure speed, and those will only be affected by the tire size.

I also dont think the speedo would be the only thing affected by changing the ratios. There are limiters in the tuning to protect the transmission, transfer case, and axle shafts from too much torque that will all be thrown off by changing the gear ratio.

 

Its actually a very easy calculation for the PCM: Engine RPM divided by transmission gear ratio divide by rear gear ratio multiply by tire circumference . Then some unit conversion factors equals speed being traveled. The fact has been tested and proven. Changes to tire circumference and gear ratio WILL correct the speedometer. One member I worked with got to less then 1 mph variance from GPS speedo.

The issue with using wheel speed sensors (however accurate it can be) is if one or more fails you would lose speedometer read out. Can you imagine what the speedo would do when traction control or ABS fully kicks in if using wheel speed sensors.

 

But you aren't changing just the gear ratio, we are changing both tire circumference and gear ratio to get an equivalent combination to what the OP actually has. Let's use his numbers to prove it out.

Chlidog's truck:
gear ratio: 3.55
Tire size: 295/70/18 which has a corrected tire circumference of 2623mm.

Let's ratio these:

2623/3.55 = 738.87

Proposed values for his BCM (from above):
gear ratio: 3.31
Tire size: 2444mm

Ratio theses:

2444/3.31 = 738.36

The values are almost identical. So what that means is a gear ratio of 3.31 and a tire circumference of 2444mm will tell the computer the same thing (speed wise at the road) as a 3.55 gear ratio with 2623mm circumference tires. Within the ranges of gear ratio and tire circumference that are accepted by the PCM there will be many combinations that would yield the same corrected speedo results. Because the ratios are the essentially the same any torque monitoring or limiting calculations that the PCM would be concerned with will not be affected.

 

so basically I change my gear ratio to 3:31 and tire circumference to 2444 mm and I’ll have no issues? Would I get the same error message?

 

The same thing as a transmission mounted sensor?

You ignored my other comment about all the other calculations the gear ratio change would affect. How about we just tell people to do it the right way.

 

Apparently, according to my 2021 3.5 tune, the PCM should accept up to 2678mm

 

After a PCM relearn and clear your codes, you should be close on your speedo and the error should be gone. That is assuming I was close on guessing your factory BCM AS BUILT data. If you can go to the website I posted the link to, enter your VIN and download your AS BUILT files. Go through the BCM file and confirm if my guess was right - unless you made a copy of your AS BUILT data before you tried to change the tire circumference - in which case you can get the info from there.

 

I did answer this comment. I can not think of any parameter that would use the gear ratio only. Any parameter I can think of would use the gear ratio and tire size together to monitor/calculate/execute something. That combination (as I calculated in my response) has been kept the same with what I am suggesting.

 

Put the circumference back to stock and use Sparts tire size calculator.

From the calculator:

726-15-01: xxxx xxxx 0Axx
726-15-02: 54xx xxxx xxxx

Write it, then go in and clear the DTCs.