Hello everyone, this is my first post so go easy on me but I’m having some troubles I can’t resolve. I put 295/70R18 on my truck and I changed the tire circumference to 2610 mm and it’s really close but I have the lovely error code for the PCM. I did relearn PCM and cleared all DTC’s but it is still present. Should I try it in hex or any better ideas would help greatly. Thank you!

 

FIrst - what year truck do you have? What gears are in the truck (are they the factory set)? What size tire did your truck come factory with?

From my experience with changing tire circumferences with Forscan, 2610mm circumference is outside the range the PCM will accept - hence why you keep getting the error. You will need to "adjust" the gear ratio as well.

 

my truck is a 2023 and factory size was 275/65r18

 

What rear gear ratio do you have?

Or post here your lines 726-15-01 and 726-15-02 from your BCM As Built data found with Forscan.

 

sorry my ratio is 3.55

726-15-01 C104 0163 0A75
726-15-02 4316 898F

 

Dont change the gear ratio. It has no effect on the speedometer.

I dont know the situation on the newer trucks, but as stated above, 2610 could be outside of the range that the PCM is willing to accept from the BCM. If that is the case, then the only option would be to use a tuner to increase the allowable size in the PCM.

 

Are these the original factory lines or are they your most recent attempt at correcting the speedo? I ask because I get a tire circumference of 2623mm from this line. If you did not save your original As Built files before making changes you should be able to download the originals here:

https://www.motorcraftservice.com/AsBuilt

 

I will respectfully tell you mass-hole that you are incorrect and that changing the gear ratio in the BCM will help fix the speedo. I have done these calculations with other members here who had newer trucks (2016, 2020, and 2024). The method worked in all cases, just the line in the BCM changes with each generation - '11-14, '15-20, and '21-24 ( I haven't tried it with a 2025 yet).

Your insight into the allowable ranges for gear ratio and tire circumference were the key bits I needed to figure this out and I thank you for that. But what I will propose for Chlidog will work.

 

I did recently try it in the calculator and that is the values it gave me with GPS and my speedometer is spot on. Is it a problem leaving that DTC in the BCM?

 

Leaving it in doesn't hurt the truck. The PCM when you did the relearn wouldn't accept that value - therefore you get the error. The PCM will revert to the factory number is has hard coded in it, which is why your speedo didn't change.

The factory BCM AS BUILT lines are only needed to understand the factory correction factor applied to the tire circumference. We carry that factor through to the new calibration values. That's all its needed for.