How to use FORScan to adjust tire size

Car configuration discussions and experience sharing
kevinmk_37
Posts: 2
Joined: Sat Dec 18, 2021 7:12 pm
Vehicle: 2019 F250 Lariat 6.2L Gas

Re: How to use FORScan to adjust tire size

Post by kevinmk_37 »

2019 F250 Lariat

Under the simple editing method in current version of FORSCAN(2.3.46), there are two lines listed:

tire circumference,mm 2631
tire size 275/65R 20"

For inputing the circumference in mm for the new tires, I could enter it on the first line per the directions above. Alternatively under tire size there is an entry for my tire size ( 265/70R17)-- Although the entry has about 100mm smaller circumference then what several calculators give me for my new tires.

Does it make a difference where to change the tire circumference (in tire Circumf or tire size ?
kevinmk_37
Posts: 2
Joined: Sat Dec 18, 2021 7:12 pm
Vehicle: 2019 F250 Lariat 6.2L Gas

Re: How to use FORScan to adjust tire size

Post by kevinmk_37 »

Today I tried both methods to change my tire size to see which one worked the best.

Original tire size 275/65R20 (with a circumference of 2631mm)
New snow tires 265/70R17 (with a circumference of 2522mm)
Before any changes my speedometer read 10% higher than actual speed reported by my phone GPS when I had my snow tires on the truck.

First method:
Per the article at the beginning of this tread, I edited the Tire Circumference, mm under the easy editing method. I have calculated the circumference of my tire on several tire size calculators and changed the Tire circumference to 2522mm.

Results: Truck speedometer still read about 10% higher than actual speed. After my test drive I went back into FORSCAN and noticed the Tire Size entry now reported "Incorrect value or not configured". The tire circumference still reported the new size that entered - 2522mm. No DTCs were thrown from my test drive.

Second method:
I changed all values back to their original values did a test drive and no DTC thrown. Then I changed the tire size entry from 275/65R20 to 265/70R17(from the list offered). I noticed that the list of choices also had a circumference value next to each tire size. For my tires (265/70R17) FORSCAN listed a circumference of 2440 (if I recall correctly) which is about 100 mm less than what several online calculators listed for these tires. I took a quick peek at the tire circumference following the tire size being changed and noticed the tire circumference was now set to 2440 just like the reading shown in the tire size list.

Results: After a test drive on the highway (Cruise control at 66mph), I noticed the speedometer was reporting 1 - 2 mph faster than my phone GPS. This was much better than pre change where the speedometer would show 6 to 7 mph faster than my phone gps when travelling 66mph.

Overall I am happy with the results and feel the speedometer is reading accurately enough for me.
tesla123
Posts: 1
Joined: Tue May 03, 2022 12:37 am
Vehicle: 2016 f250 6.7 diesel

Re: How to use FORScan to adjust tire size

Post by tesla123 »

whats up everyone new to the forum but been a technician for years, so i have a 2016 f250 on leveling kit i just put rims with 37" on them i tried almost everything to calibrate the new tire size but everytime i get the check eng light and the trac light and it wont reset until i put it back to stock i searched the forums found the procedure did it but still the same thing and also my speedo is off by 5mph not sure what i am doing wrong or not doing any help will be appriciated.
h00ktern
Posts: 1
Joined: Sun Mar 24, 2019 1:32 am
Vehicle: F150, 5.4, 2009

Re: How to use FORScan to adjust tire size

Post by h00ktern »

Seems to be a common issue. My current experience with my brother's '19 F-250 4x4, 37" tires:

Input the 126-12-01 hex change using the calculator. Relearn PCM. Success. Relearn TCM fails. Subsequently get the DTCs.

Using PCM configuration to pick the largest tire circumference choice, Relearn PCM, no DTCs.

Best I can ascertain is the DTCs come about from not being able to relearn the TCM, the tire size is outside the current database in the module(s,) and the tire circumference falls outside min/max standards of current module programming.

I have not found a solution in Forscan, but think I recall seeing a post where someone received a beta/patched version of Forscan whick allowed him to write to the TCM.

I'm open to the solution if one exists...
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