Changing tire size using FORScan

[AZ_Ryan]

I just bought a new set of PP wheels for my '24 GT premium. I stopped by my deal today to book an appointment for them to change the tire size using Forscan, but they told me the software has changed and they can't do it anymore without me vehicle throwing codes.

The same dealer did it for me on another vehicle last spring without issue, so I'm perplexed.

Are they just pulling this out of their ass or is this an actual thing on the S650s?

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[gtstang462002]

Haven't tried changing tire size on mine with the Forscan, but definitely looked at the option in the menu. If someone else doesn't report back, I can try on on mine and let you know the results in a day or 2.

 

[Thuggs]

You can calibrate the speedo through the dash settings maybe you don’t need for scan

 

[AZ_Ryan]

You can calibrate the speedo through the dash settings maybe you don’t need for scan

You can't calibrate the speedo through the dash settings. You have to change the tire diameter code.

 

[Thuggs]

Well on mine I was able to change the speedo calibration to match gps speed thru the dash aus spec cars are different to US ones might be the difference

 

[AZ_Ryan]

Well on mine I was able to change the speedo calibration to match gps speed thru the dash aus spec cars are different to US ones might be the difference

Interesting. Where in the settings did you have that option?

 

[Thuggs]

Interesting. Where in the settings did you have that option?

It was in the settings but cannot remember where still navigating the screens but I think it was percentage based
Seems to be big differences I have folding mirrors which apparently US spec cars don’t get
We don’t get any of the base models it appears

 

[npole]

After have changed the size (via ForScan) you need to reset (re-learn) the PCM and TCM (if present) modules, otherwise the car will throw errors and some functions will work erratic (cruise control).

It's interesting that feature to adjust the speedometer via the interface, if it's there (AUS) then it's in the APIM and it could be potentially activated by anyone... but I never heard about it.

 

[Scrape]

It was in the settings but cannot remember where still navigating the screens but I think it was percentage based
Seems to be big differences I have folding mirrors which apparently US spec cars don’t get
We don’t get any of the base models it appears

I wish we had folding mirrors. I wonder how much they are to bring here in Canada.

 

[AZ_Ryan]

After have changed the size (via ForScan) you need to reset (re-learn) the PCM and TCM (if present) modules, otherwise the car will throw errors and some functions will work erratic (cruise control).

It's interesting that feature to adjust the speedometer via the interface, if it's there (AUS) then it's in the APIM and it could be potentially activated by anyone... but I never heard about it.

Thanks. Not sure it's worth messing around with then. I want my speedo accurate, but in my experience resetting the PCM and TCU always seems to lead to a bug or glitch. Everything is working perfectly on my car now (knock on wood). We'll see how far off it reads. Tire size calculator says the 275s are a good amount taller and the speedo will read 1.5-2 mph slower than actual at 70 mph.

It's too bad the cars speedo is pretty spot on from the factory. All the S550s speedos read 1-2 mph faster than actual from the get go.

 

[gtstang462002]

Thanks. Not sure it's worth messing around with then. I want my speedo accurate, but in my experience resetting the PCM and TCU always seems to lead to a bug or glitch. Everything is working perfectly on my car now (knock on wood). We'll see how far off it reads. Tire size calculator says the 275s are a good amount taller and the speedo will read 1.5-2 mph slower than actual at 70 mph.

It's too bad the cars speedo is pretty spot on from the factory. All the S550s speedos read 1-2 mph faster than actual from the get go.

Use Waze with Android Auto or Caraplay. It displays a GPS speedometer on the map that you can use to validate your current speed on the dash. It probably won't be off far enough to care.

 

[npole]

Not sure what to what it would lead having PCM and TCM to relearn the data... it's more "dangerous" playing with the actual addresses IMO... but yeah, normally you want to adjust the tire size only if you really changed your tires size (or.. like me.. to see if you can override the speed governor to fool it with a lower speed ).

 

[AZ_Ryan]

Use Waze with Android Auto or Caraplay. It displays a GPS speedometer on the map that you can use to validate your current speed on the dash. It probably won't be off far enough to care.

Yes. That's how I check it. But the math doesn't lie.

 

[gtstang462002]

Yes. That's how I check it. But the math doesn't lie.

The speedos in cars are probably more precise today than they ever have been, but in years past they used to vary by as much as 5% from actual ground speed. I don't know why they don't just display GPS speed on the dash and have referenced wheel speed as a backup option for lack of GPS signal (tunnels and such). Then tire size/wear would be less of a concern affecting the displayed speed.

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