Speedometer off after diff (pumpkin) swap

[MrMike]

I went from a 3.31 / Trac lok to a 3.73 / Torsen.

Car was originally a non-PP car equipped with 235/50R18 tires and now it has 285/35R19 tires. Not a huge difference, 1mph at 60.

When I first installed the diff, the car pulled power over 3k rpm in 2nd, 3rd, and 4th gears. Did not bother trying 5th or 6th, but I would assume the same issue would persist.

Based on what I read here, in Forscan under BdyCM I changed the axle ratio to 3.73 (hex 0175) and the tire diameter to 2072 (hex 0818) per the spreadsheet in this thread. Then I did the PCM re-learn.

Car runs fine, no pulled power. I can do a 1-2-3 pull w/o issues.

The issue is that my speedometer is 5mph under. Changing the tire diameter to 0818 and re-learning the PCM will have it work for a bit, then it will go back to being 5 under. The values in Forscan do not change back; the speedometer simply no longer reads the correct speed. I am using my phone for verification. For comparison, my Bronco is within 1mph of the phone in the same areas at the same speeds.

Car does not have a tune.

Any suggestions?

I am thinking I will need to get a Procal but was hoping this was fixable with just Forscan.

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[Cobra Jet]

Pass along info:
Searched and found some threads relating to S550 rear gear changing and tuning. What is in the thread content is not all related to a specific S550 model or year, BUT what is discussed may help to answer some questions or maybe provide some more guidance:

https://forum.hptuners.com/showthread.php?75313-2018-Mustang-Gear-Ratio-Change

https://forum.hptuners.com/showthread.php?64135-2015-Mustang-GT-Speedometer-Calibration

https://forum.hptuners.com/showthread.php?68888-17-Mustang-GT-Traction-Control-Advancetrac-Question

 

[MrMike]

https://forum.hptuners.com/showthread.php?64135-2015-Mustang-GT-Speedometer-Calibration

This thread makes me think a tune or device (like the Procal) is required. Unfortunate.

 

[ice445]

This thread makes me think a tune or device (like the Procal) is required. Unfortunate.

It is on the newer cars unfortunately

 

[MrMike]

It is on the newer cars unfortunately

I was afraid of that. Unless somebody pops in with something I obviously missed, I'll be calling Ford Performance on Monday to talk about the Procal.

 

[Mike Pfeifer]

I bet you could use pcmtec for this, if it’s all you want to do, might be more cost effective in the end.

 

[MrMike]

I bet you could use pcmtec for this, if it’s all you want to do, might be more cost effective in the end.

I googled PCMTec but the quick hits all look like it's in AUD. Am I on the right site? It looks like it needs a different cable from Forscan. https://www.pcmtec.com/pcmtec-editor-protuner-edition-usvp1

I can get a plain Procal 4 to use for debugging and tweaking for $400. Separate display in car could be nice too.

 

[Mike Pfeifer]

It does show in AUD, but I think it will convert at purchase or something like that. It’s been a couple months since I looked at it. It does require a different cable, there is one that’s like $140 I think, and the pcmtec conversion comes out to like $120 if I remember correctly. (Assuming you only want to use it on the one single car and don’t want custom OS.)

 

[Mike Pfeifer]

@tdstuart would have some better recent experience with what is needed specifically to get it working. He just went through it a few weeks ago.

 

[MrMike]

It does show in AUD, but I think it will convert at purchase or something like that. It’s been a couple months since I looked at it. It does require a different cable, there is one that’s like $140 I think, and the pcmtec conversion comes out to like $120 if I remember correctly. (Assuming you only want to use it on the one single car and don’t want custom OS.)

Good info.

I looked at HPT too but their MPVI3 with a credit is not cheap either. Is VCM Suite included if you buy the device?

I don't own a Windows laptop w/a functioning battery currently, just an old ThinkPad that needs to be plugged in.

I'll sleep on it but I like the ability to use the Procal as a stand alone diagnostic / tweaking device. Also like that FP will back me up if it doesn't work out for some reason.

 

[tdstuart]

@tdstuart would have some better recent experience with what is needed specifically to get it working. He just went through it a few weeks ago.

I had the same gear ratio so I didnt have to change anything. There is a special function for speedometer calibration that hptuners has, same type of function like crank relearn. PCM tec doesn’t have functions like that as far as I know but you can change speedometer settings in the tune file.

Ive tried playing with the speedometer calibration but I would need to watch the digital readout on a logger in real time to tell if it’s accurate or not.

The car has some learning features for the speedometer and tire sizes. I’m not sure how it identifies that it needs to make a change and how it determines what the correct values should be.

Later tonight I can pull up the options you would have with pcm tec. You could probably disable the learning but I don’t know if this is the technically correct route or if there may be another value you have to change.

 

[Mike Pfeifer]

Good info.

I looked at HPT too but their MPVI3 with a credit is not cheap either. Is VCM Suite included if you buy the device?

I don't own a Windows laptop w/a functioning battery currently, just an old ThinkPad that needs to be plugged in.

I'll sleep on it but I like the ability to use the Procal as a stand alone diagnostic / tweaking device. Also like that FP will back me up if it doesn't work out for some reason.

All good points. Just wanted to toss something out there that you might not have considered is all. I’m sure whatever way you go will get you where you want to be. It’s too bad that forscan alone cannot do it.

 

[ice445]

I had the same gear ratio so I didnt have to change anything. There is a special function for speedometer calibration that hptuners has, same type of function like crank relearn. PCM tec doesn’t have functions like that as far as I know but you can change speedometer settings in the tune file.

Ive tried playing with the speedometer calibration but I would need to watch the digital readout on a logger in real time to tell if it’s accurate or not.

The car has some learning features for the speedometer and tire sizes. I’m not sure how it identifies that it needs to make a change and how it determines what the correct values should be.

Later tonight I can pull up the options you would have with pcm tec. You could probably disable the learning but I don’t know if this is the technically correct route or if there may be another value you have to change.

Just pointing out that the way these things work is different on the 18-23 cars compared to the 15-17 like you have

 

[MrMike]

Just pointing out that the way these things work is different on the 18-23 cars compared to the 15-17 like you have

Agree for anyone else who may find this thread, but mine is a 16.

 

[MrMike]

I had the same gear ratio so I didnt have to change anything. There is a special function for speedometer calibration that hptuners has, same type of function like crank relearn. PCM tec doesn’t have functions like that as far as I know but you can change speedometer settings in the tune file.

Ive tried playing with the speedometer calibration but I would need to watch the digital readout on a logger in real time to tell if it’s accurate or not.

The car has some learning features for the speedometer and tire sizes. I’m not sure how it identifies that it needs to make a change and how it determines what the correct values should be.

Later tonight I can pull up the options you would have with pcm tec. You could probably disable the learning but I don’t know if this is the technically correct route or if there may be another value you have to change.

At the risk of being boring, this makes me lean even further to the Procal lol. I've read almost all of your custom tuning thread, and that is not a rabbit hole I want to go down.

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