Correct your speedometer error - no flash or tune required

[Spart]

I think the wifi one has that issue

And there's no way I know of to check that the PCM relearn was successful, unless your DTC goes away and then you know for sure.

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[Bespinoza66]

Thanks so much .
I will give it a go !

https://www.mustang6g.com/forums/threads/modifying-as-built-data-guide.61773/

That's the guide I used and it worked very well. Read it thoroughly and you should be good. Purchase the Bluetooth version and make sure to use a PC, not apple.

 

[Bespinoza66]

And there's no way I know of to check that the PCM relearn was successful, unless your DTC goes away and then you know for sure.

Yeah, forscan seems to be the only way to know without going through a much more expensive service

 

[Spart]

Yeah, forscan seems to be the only way to know without going through a much more expensive service

I don't think FORScan can read the values that the PCM relearns during the procedure. Hence, no way to verify.

 

[Bespinoza66]

I don't think FORScan can read the values that the PCM relearns during the procedure. Hence, no way to verify.

As long as the code goes away and the car doesn't explode, it may be safe to assume it does lol

 

[Spart]

As long as the code goes away and the car doesn't explode, it may be safe to assume it does lol

Right. But what I'm saying is, if the relearn was maybe successful and something else may be going awry, you have no way to confirm what the problem is when you do still have a DTC present.

 

[FreePenguin]

I am going from 255/40/19 on 19 to 275/40/19 tomorrow on 9.5. I am going to attempt to change the one line of code for the change, is it still safe to do now a days? following the guide in original line or has there been negative failures?

im not doing transmission / ratio changes. Just the tire size change.

 

[SAY WHAT]

You should not have to with those sizes.

 

[FreePenguin]

I read there’s something like 4% off
If not I will disregard interested to hear opinion, I will of course compare to gps first before I tinker

 

[SAY WHAT]

you will only be off like 2 mph at 80mph anything under will only be 1. Tbh performance pack cars run 275/40/19 in the rear where the speed measured.

 

[Spart]

Tbh performance pack cars run 275/40/19 in the rear where the speed measured.

Performance Pack cars are also factory calibrated differently for that tire size.

 

[FreePenguin]

im a non PP ecoboost, but if its only off by like 1 mph, I am not going to bother, in fact I kinda like a little 1mph cushion at 70mph (our speed limit is 65-70) but if it was like 4mph id of had to change. if I go 285/35/19 will it be the same as well?

reason I ask is im test fitting 275 on my wheels today to see how they look, if doesn't give the curb protection I need for rim, may add 285 instead.

 

[Spart]

im a non PP ecoboost, but if its only off by like 1 mph, I am not going to bother, in fact I kinda like a little 1mph cushion at 70mph (our speed limit is 65-70) but if it was like 4mph id of had to change. if I go 285/35/19 will it be the same as well?

reason I ask is im test fitting 275 on my wheels today to see how they look, if doesn't give the curb protection I need for rim, may add 285 instead.

That's not how it works.

Going from 255/40R19 to 275/40R19 is a 2.3% gain in circumference. When you go up in circumference, your speedometer reads slower, not faster.

When your speedometer tells you you're doing 70, you'll actually be doing 71.6 or so, not 69 like you're thinking.

 

[FreePenguin]

then I will be changing my calibration, I stick my speedo right on 70mph for cruise, so the question, by finding my new value via hexadecimal and changing that 1 line of code, its been pretty foolproof right? no engine codes/issues? I know I read all the pages of this and it seemed people changing the parameters for the gearing/transmission was having DCT's etc. but I dont mind doing the forescan change (I use forescan a lot and takes 2 seconds) just dont want issues to pop up with non erasable codes/or popping other issues like gimp mode

 

[Spart]

then I will be changing my calibration, I stick my speedo right on 70mph for cruise, so the question, by finding my new value via hexadecimal and changing that 1 line of code, its been pretty foolproof right? no engine codes/issues? I know I read all the pages of this and it seemed people changing the parameters for the gearing/transmission was having DCT's etc. but I dont mind doing the forescan change (I use forescan a lot and takes 2 seconds) just dont want issues to pop up with non erasable codes/or popping other issues like gimp mode

You'll absolutely get a DTC if you don't follow the instructions and do a PCM relearn after setting the size in the BCM.

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