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Calibration for large tires with Forscan (No CEL)

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Calibration for large tires with Forscan (No CEL)

  #1  
Old 05-16-2018, 02:57 PM
ghatzie
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Calibration for large tires with Forscan (No CEL)

Ok apologies to all and big shout and thank you to SIL17SD for catching my mistake here. I had worked this out, drove my truck for a few days to test, it worked well then when I posted I reversed the axle ratios in my head and divided the wrong way to get the tire correction factor. I sincerely apologize to everyone, this has been corrected on 5/20/18, and it works properly now!! The text below is fixed as well as the excel calculator


Hi All,

Theres been multiple posts about how to correct for larger tires using Forscan however some larger sizes seem to throw codes. I believe Ive found a workaround to correct without getting a CEL.

in the BCM module, address 726-12-01 holds this information in the format XXXX-YYYY-ZZZZ which are of course stored in hex. Here is a quick breakdown of the values:
XXXX = rear axle ratio (x 100) in hex. So:
3.31 = 331 (decimal) = 014B (hex)
3.55 = 355 (decimal) = 0163 (hex)
3.73 = 373 (decimal) = 0175 (hex)
4.10 = 410 (decimal) = 019A (hex)

YYYY = tire circumference in mm, value converted to hex and written.

ZZZZ = no clue at the moment.

Now it seems that values for YYYY over 0ABA will cause a CEL. However the rear axle value (XXXX) can be modified to reduce the tire circumference value (YYYY) such that the overall distance per rotation is ultimately correct.

For example (my truck), I have 38x13.5r22 tires with a 3.55 rear axle, 38" tire diameter gives a circumference of 119.3in or 3030mm.
3030 in hex = 0BD6.
Writing 0BD6 to YYYY will properly adjust the speedometer (and odometer) however it will throw a CEL.

By changing my axle ratio (in XXXX) to 3.20 will allow me to use the ratio of my actual rear axle to the modified value to adjust the tire size (and yield the same distance / rotation value).

So if I divide 320/355 = 0.901

I multiply my tire circumference (3030) by 0.901 = 2733 or 0AAD (less than the 0ABA limit which will throw a CEL).

I can now write the following values to 726-12-01:
XXXX = 0140
YYYY = 0AAD

And get the correct adjustment factor with no CEL.

I do not believe (so far) that the rear axle value here is used to do anything else but determine the correct speed / mileage but I could be wrong so use at your own risk (much like everything we do with Forscan)

Happy lifting and big-tiring!!
(and with regards to the title of this post, technically were adjusting not calibrating but thats what everyone seems to want to call it...)

Ive added an excel calculator to help, should be pretty easy to use but if you have questions just post.
 
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Old 05-16-2018, 03:16 PM
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This makes sense, seeing as the dealership set me to 0163, which according to you is 3.73s even though I have 3.55s
 
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Old 05-17-2018, 05:49 AM
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3.73 setting in a 3.55 truck would be the wrong way to adjust, unless you put smaller tires than factory on your truck.

What is the value for 3.30? if I did the math right that would be correct for a 3.55 truck with 37s.

3.30 with stock 34" tires is almost identical rpm/mph to 3.55s with 37s
 
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Old 05-17-2018, 06:14 AM
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Originally Posted by Sil17SD View Post
3.73 setting in a 3.55 truck would be the wrong way to adjust, unless you put smaller tires than factory on your truck.
Edit, I just noticed his tables are 1 off. 0163 is 3.55.
 
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Old 05-17-2018, 06:24 AM
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Can you tell me what value to try for 3.30s?

TIA
 
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Old 05-17-2018, 08:23 AM
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Originally Posted by briangp View Post
Edit, I just noticed his tables are 1 off. 0163 is 3.55.
Originally Posted by Sil17SD View Post
Can you tell me what value to try for 3.30s?
Sorry guys, and good catch, table is corrected and I mistyped 3.31 values are correct now and you can find 3.31 in the table. The methodology for calculating the values is correct though.
 
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Old 05-17-2018, 08:28 AM
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Originally Posted by Sil17SD View Post
3.73 setting in a 3.55 truck would be the wrong way to adjust, unless you put smaller tires than factory on your truck.

What is the value for 3.30? if I did the math right that would be correct for a 3.55 truck with 37s.

3.30 with stock 34" tires is almost identical rpm/mph to 3.55s with 37s
Hi Sil,
Correct so to accommodate larger tires without throwing codes, you have to use a larger axle ratio then reduce the tire size proportionately (in order to get it under the ABA limit so you dont get a CEL). So technically both values (axle size and tire size) are incorrect that they do not match what is really on your truck, but they make the math work in terms of distance per rotation which allows the truck's speedometer to reflect your actual linear speed.

I can tell you im running this on my truck now and it seems to be 100% correct with no other issues.
 
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Old 05-17-2018, 08:32 AM
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Ok thanks, stupid question is that in Bdycm or BCMB
 
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Old 05-17-2018, 08:39 AM
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Originally Posted by Sil17SD View Post
Ok thanks, stupid question is that in Bdycm or BCMB
BodyCM as built. You won't find the 726-xxxx codes in any other module
 
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Old 05-17-2018, 09:12 AM
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Yep I saw that. I made the change just to the ratio and it's still off but real close. 60 on the speedo is 62 gps.

So now I just need to go read up on the tire values and start playing with that.
 
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Old 05-17-2018, 09:19 AM
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Originally Posted by Sil17SD View Post
Yep I saw that. I made the change just to the ratio and it's still off but real close. 60 on the speedo is 62 gps.

So now I just need to go read up on the tire values and start playing with that.
Not sure what values you're using but I'd say walk the tire number up a few at a time and see where you end up. If you run out of room to move it up, you'll have to ''shift gears' to another axle ratio.
 
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Old 05-17-2018, 09:23 AM
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I'm running stock tire setting and doing the math says I need to adjust it to 35.5" diameter.

I have to go figure out how to convert the rollout numbers to hex now.
 
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Old 05-17-2018, 09:39 AM
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Originally Posted by Sil17SD View Post
I'm running stock tire setting and doing the math says I need to adjust it to 35.5" diameter.

I have to go figure out how to convert the rollout numbers to hex now.
Well i didnt get the point about you running stock tires and needing to adjust it but assuming you are actually have 35.5" tires mounted and need to adjust...

Lets go through the math real quick. 35.5" tire diameter tives a 111.5" circumference (35.5 x pi) = 2934 mm which is 0xB76 (hex) which is too large.

To convert from decimal to hex the easiest way is just to type it in in google or use the programmer layout of the windows calculator, enter the number in Dec and switch to Hex.

So youll have to choose a different axle ratio and follow the instructions above. If you need help just post here (give me your current axle ratio) and ill help you figure out values that work.

If I have some spare time at some point ill make a calculator in excel to take the maths out of it...
 
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Old 05-17-2018, 10:08 AM
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Sil give me your tire size and stock axle ratio please. You have really given a clear answer. I'll do it for you.
 
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Old 05-17-2018, 10:08 AM
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Awesome to see that this actually works. I had thought about trying this at one time but got side tracked.

Two things to ponder....
I initially tried using the tire vendor provided height as a reference for my calculations. But they always seemed off. Then someone asked me to try the "running tire size" and that seems to work perfect for me. Running/loaded tire size is determined by measuring your tire radius from ground to center cap, then doubling this number. My 37's are claimed to be 36.6" tires on the nitto website, but the loaded tire at 70psi is closer to 35.6".

I was helping one person via PM adjust his tire size and we tried several values greater than the ABA maximum, and he never threw a CEL. Have always wondered if he typed something wrong or if all Canadian based trucks don't have this limitation in the TCM. The check engine light comes from the value the TCM learns via the BCM (what we are programming). But typically the TCM has a soft coded limit and once our tire size goes greater than that value the CEL is triggered via a mismatch.
 

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