2022 Timberline Tire size adjustment with forscan

[mtrosclair]

So, I finally got new tires and I need some help because for some reason I am not understanding how to figure out getting the tire size in forscan correct.

Long story short I decided on the Firestone Destination XT and they were not available in the stock size of 265 70 r18 with the white letters, they were available in 275 70 r18 and the shop absorbed the cost of the slightly larger tire. I assumed that changing the size value in forscan would be easy but I have not had a good result yet. The stock circumference in MM was 2580 (I believe, I wrote it down but don't have it with me), the outer circumference of the new tire is listed at 33.2 which is 2649.2 MM, I changed it in Forscan and the speedo is now 3.5 mph high. With the old tires it was dead on verified by GPS. I don't want to randomly start changing numbers but I cannot wrap my head around how to approach this and google hasn't been much help. So I am looking for advice, and I need to get the thing cleaned up and take some pics for the photo tax.

Thanks

 

[mtrosclair]

So I was doing it wrong (no surprise) I needed to do a PCM relearn after changing the circumference value in the plain english part of the BdyCM however the max value it will take is 2610mm which is not enough to completely remedy the speedo disparity. it is now 2 mph higher than actual.

It looks like there is a way to fix this fully with the as built portion but I need to do abt more research before i move forward so I am going to post in the FORSCAN forum.

 

[TheDoug]

Did you find an answer for this?

 

[mtrosclair]

Did you find an answer for this?

Unfortunately it looks like the only answer may be to get a tuner which I am not going to do for a 3 mph discrepancy. I don't understand the "as built" part of Forscan to go mucking about so this may just be it.

 

[TheDoug]

I have used ForScan for lots of other things, but never the speedo.

It sounds like you were doing it correctly if you found the setting.

BCM -> "click play" -> Select Tire Circumference -> then enter value -> "write data"

check this video out and see if it helps (its for a Bronco but steps should be same)

https://www.youtube.com/watch?v=aB8QKx6H9E0&t=98s