FORScan and Tire Size Changes

[Bloodham25]

I did mine today with Forscan. Went into BdyCM and changed value from 2178 to 2080. I did not get a check engine light and the speedometer appears right via GPS speed.

I'm about to do mine.

Basically I just take whatever the Number #2 tire size is (In circumfrence) and enter that in with ForScan?

I'm going from 275/40/R19 rear to 285/35/R19 rear.

So it would be 2207 --> 2141 (This is the number I enter)

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[bent98]

I followed the instructions on the Forscan forum but there is no way for me to Initialization/Relearn vehicle data functions for PCM and TCM module as the option for the TCU is not available in the config and programming tab. It did take the updated value 2299mm for my 305/45/18 MT Street R's no problem. (stock value was 2132mm)

After making changes to the factory software, I flashed back to my LUND tune which doesnt overwrite the tire values. the MPH seems to be ok but i got a lightning bolt. I turn off car and turned it back on and the lightning bolt disappeared. I went in with my OBD reader and there was no codes.

I'm not sure what to do. attached is the instructions.

 

[LikeMike]

Reviving this thread because it's the top one on google for newish results and I did this yesterday on my 2022 Mach 1 with no issues.

In the rear I went from 275/40 R19 to 325/30 R19. The OEM value was 2178mm but the tiresize.com calculator said 2207 for the OEM tire. The reason for the difference is probably that tiresize is getting it from a specification where Ford is dyno results within 5% at recommended tire pressures. I never tested which was more accurate. Instead I did 2178/2207 = 0.98686. Which means that when the calculator told me I was now at 2129mm for the 325 tires, it needed to be lower. I did 2129mm x 0.98686 = 2101. Prior to adjustment it was off greater than tiresize calculated likely due to the addition of OEM calibration error. Was off 3-4 mph, I was moving slower than the Speedometer was measuring.

Afterwards I input that 2101mm into Forscan and wrote it to the module. The car reset itself and I didnt need to do anything else. On a android, iPhone, and radar, I was registering about 1-0.5 mph. As in at 60mph on the speedometer but I was registering 59mph on GPS. I will make sure my tires are at the correct pressures and if so reduce it a few millimeters (1%) to around 2080 as I'd like to dial it in a bit more. So anyways you can see how tiresize.com had it at 2207 but the true value is less than 2101. In other words you are putting in a relative value for diameter not the absolute, and even then if you really want it spot on it's a bit of guess work.

Didnt have to manually reset or relearn any modules, no codes, etc. Was very straight forward. On the latest version of forscan using the recommended ODB cable.

Forscan version 2.3.55

 

[Zathras]

For small** changes in tire diameter/circumference, is any adjustment necessary if you don't care about the speedo being accurate? For example if I go to a 2% shorter tire from stock, it's only going to affect the speedo 1-2 mph at "normal" driving speeds. I'm more concerned about whether the shift points (A10) will still be right (not hit rev limiter, etc.)

** I often see people say a change of "less than 3%" is OK, but I've never seen any documentation on that.

 

[WildHorse]

I get the P160A. No CEL, no wrench. Accurate according to GPS.

You gotta do whatcha gotta do.

Especially when you go from a as-built 3.31/235 tire to 3.73/285 tire.

 

[AZ_Ryan]

So is there a consensuses on what the forscan value should really be when programing a non HP M1 to HP tire spec?

I had the dealer do mine and its still off a little.

 

[MidnightStars]

So is there a consensuses on what the forscan value should really be when programing a non HP M1 to HP tire spec?

I had the dealer do mine and its still off a little.

If it's still off after that, use the GPS calculation method:

So is there a consensuses on what the forscan value should really be when programing a non HP M1 to HP tire spec?

I had the dealer do mine and its still off a little.

Use this:

I still have a PCM code (since BCM value is out of range), but no check engine light

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