I think I have tried it while starting the vehicle too but it's been a while since my last attempt.

Battery is probably dead now anyway, haven't started it in a week. ha

I'll give this a try soon...weekend maybe.
I am one of the fortunate ones that is currently working. I'm a little busy this week -will see about making time on the weekend by which time I'll have totally forgotten about this.

 

Forscan can change tire size. I did it on my truck without any issues. I believe the setting was in the BCM menu.

 

;;

 

I'm afraid of that, I know I'd screw something up, I have next to no skill or patience with electronics, but, I do know some guys that do. Likely I should just get together with them.

 

You’re not writing code or hacking the CIA mainframe. The menu is in plain english. All it’s asking for is the tire circumference in millimeters. Take your new tire diameter in inches, convert to millimeters (inches x 25.4), multiply by pi (3.14). This should give you a number in the 2000-3000mm range.

Example:
35” new tire diameter x 25.4 = 889mm tire diameter
889mm diameter x pi (3.14) = 2793mm <<< this is the number the computer wants.

Alternatively, you can take the stock circumference number already in the computer, multiply by the new tire diameter in inches, then divide by the stock tire size in inches. This will use the ratio of new to old tire sizes to scale the circumference number accordingly.

Example:
2546mm (already in computer) x 35” new tire size = 89,110
89,110 / 31.9” stock tire size = 2793mm <<< this is the number the computer wants.

If you have access to a GPS device capable of measuring speed, you can skip these steps and go straight to the one that will get you dead nuts accurate.

1. Install the new tires.
2. Get on a flat straight stretch of highway and set the cruise control to the highest speed you can safely hold. Let the truck come up to speed and stabilize at that speed. Let’s call it 80mph.
3. Using your GPS device, note the actual speed of the vehicle. Let’s call it 88mph.
4. Fire up Forscan, go to the BCM menu, and find the existing tire circumference value. Let’s call it 2546mm.
5. Multiply the existing value by the actual speed and divide by the cruise control speed.
6. Enter this as the new value in Forscan.

Example:
2546mm x 88mph actual speed / 80mph intended speed = 2800mm <<< This is the number the computer wants.

If you’re scared you’ll screw the programming up, write down the stock value before you change it. You can always go back if needed.