Originally Posted by ghatzie

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.

Originally Posted by workingdan

Ok, so I tried this today. I changed 726-12-01 as built from 01AE 0A47 71B1
changed to 0175-09F1 was off by 7 mph
changed to 0175-0A01 was off by 6.5 mph
When I went to put it back to stock, I noticed the third set (71B1) had changed. So now its back to stock config. I will play with it more next week.

Originally Posted by KYLongRifle

The third set of digits is the checksum you can ignore them.

Originally Posted by KYLongRifle

These are the exact same tires I'm running. I do have 3.55 gears but that shouldn't matter for what I'm about to tell you.

I use the code of ABA (largest possible without CEL) and I am spot on when the tires are set at 65psi... but
at 80 psi im about 2mph off at 80mph.

Originally Posted by workingdan

I did the ABA code last night. On the way to work this morning its within 1 mph but have only drove up to 45 mph. I will hit the highway this afternoon and see how it is at 70 mph.
Thanks

Originally Posted by Skaz

Nope, no light. Using that combination keeps it within factory values.