How to: Recalibrate Speedometer with Forscan for 2021 Bronco

[pmoore00]

So this is the exact series of events we followed. Changed the circumference in Forscan myself, then went to do the AWD relearn, which went fine. Then I tried the PCM relearn in ForScan, no luck. Took it to my dealer Spirit Ford in Dundee Michigan. They hooked it up in FDRS right in front of me ran the simple PCM relearn took 2 minutes, then we cleared the codes. No lights no issues, circumference is still where I originally set it. Been driving on it for 2 days with no issues.

That's awesome. I hope all dealers are willing to do it. I'll report back!

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[b4000]

So this is the exact series of events we followed. Changed the circumference in Forscan myself, then went to do the AWD relearn, which went fine. Then I tried the PCM relearn in ForScan, no luck. Took it to my dealer Spirit Ford in Dundee Michigan. They hooked it up in FDRS right in front of me ran the simple PCM relearn took 2 minutes, then we cleared the codes. No lights no issues, circumference is still where I originally set it. Been driving on it for 2 days with no issues.

Ok. I’m going to do the same thing.

 

[Spart]

When I try and do the PCM relearn It won't run, I says the process has been interpreted. I can do the AWD relearn no problem, Any ideas why that might happen?

This happens on 2021+ F150's and 2020+ F250/350's as well. Not sure if it's affecting newer Mustangs yet.

FORscan is not yet able to correctly initiate the PCM relearn on the newer models for some reason. It works fine on the older ones. I know this is of no help since there isn't an older Bronco that the PCM relearn would have worked correctly on, but hopefully that provides some context to the situation. I suspect it will be resolved in the next several months with a new version of FORscan, but I don't have any inside track on FORscan development.

As posted above, the tool that the dealers use to do a PCM relearn does work, so that's an option. FORscan is free software that mimics $$$$ software/hardware the dealerships use, so that's why it's behind the curve a bit. Give it time.

That’s awesome and lucky! My tire size adventure has been less satisfying, but based on all the research and poking around I’ve done I came to the conclusion that the speedo must be significantly off for a stock Black Diamond. The BCM is set to 2440mm, which is much smaller than the 265/70R17.

Just curious about the video as far as the original tire size he had for the 265/70r17s. It showed 2440 as the circ. I use the tire size calc and it shows 2520 to be the correct setting for these tires. I wonder why the initial setting was so different.

While I am not going to say that Ford isn't being sloppy and getting things wrong, I can confidently say that there is a factor missing from the video in the OP and the instructions on the FORscan.org thread linked in the spreadsheet: a tire's revolutions per mile won't exactly equal the nominal diameter of a given tire size. You can confirm this for yourself by looking at specs on TireRack, which usually provides a revs per mile figure. Tires aren't rock-hard, they're inflated with air and have some give and squish to them. This is why the revs per mile figure differs from the nominal (or even actually measured) tire circumference.

Obviously, your speedometer needs calibrated to the revs per mile, not the nominal tire size.

In my calculator spreadsheet, I account for this by setting the final value to 96.7% of the calculated nominal size in millimeters. This is consistent with what Ford does for the F150, Mustang, Fusion, and a few other vehicles I've looked at. I've not poured over enough Bronco .ab files to see what they're using there. It's actually different for every tire model, but 96.7% will get you in the neighborhood. If you don't apply that correction factor, the truck will think it has a bigger tire than it really does and the speedometer will read fast.

My spreadsheet is here, you'll have to copy/paste the link to the Google Sheet since if I make the link hot, the forum will try to embed it and you won't be able to use it:

docs.google.com/spreadsheets/d/1mWnuQZlDTUGjKjmJje1QJIR8Z2c7Kf8KBZxKnw_8EqU

Make sure to follow the instructions at the top to make a copy that you can edit. You'll need to do this on a PC, it won't work on mobile.

If you want your speedometer to be dead-nuts on, there's a procedure at the bottom of the spreadsheet to do "GPS truing" as well. That's the best method if you're OCD about it. Every different model tire of a given size will have a slightly different revs per mile, which is where the variance comes from.

Unless people are ignoring the DTC on the PCM... Several people even in this thread have said "and I just left it".

Anyway, just wanted to share in case you're like me and you can't let it go. I may *have* to let it go because there may not be an answer. Yet.

A word of warning about just leaving the P160A DTC in the PCM. That DTC is there for a reason and it's telling you the PCM is mad about something.

These vehicles rely on a number of sensors - four wheel speed sensors and a speed sensor in the transmission at least. Some Ford vehicles have a sensor in the diff too, not sure about the Bronco.

I can tell you from personal experience in a Mustang that after having corrected my tire size and then driving around with the P160A DTC, I experienced some drivability oddness that should give you pause. There was a dead spot at a certain RPM in a certain gear. When I hit that dead spot, I could floor the accelerator and the car would not speed up. Only way to get through it was to completely let off the accelerator for a beat and then mash it again.

Others (in the Mustang and F150 communities at least) have reported similar things. I think where it happens may be different for each tire size, because it's probably some mathematical formula that's freaking out once two values don't match by a certain amount. And that probably varies with how different your tire size is in your BCM vs what your PCM thinks it should be.

I'm not saying this could cause damage, it hasn't that I know of. But every Ford vehicle is a little different, and I'd hate to see a lean condition frying engines just because people want their speedometers reading correctly after fitting bigger tires.

These issues went away for me in my Mustang after doing the PCM relearn successfully.

 

[wyobronco]

I might have missed it in this thread, but I noticed the other day on waze, then on a gps app on my phone that my speedo said im going 2-3 mph slower than I was. This is on a sasquatch BD.

Is this a thing people are dealing with?

 

[level3looper]

How do these numbers effect the MPG readout on the trip meter? I just finished 3k miles with premium fuel and calculated my mpg on each fill up based on gallons to refill into miles driven from the trip meter. I've consistently got about 1.3 mpg less than what the trip meter says. I have the Sasquatch tires and know the circ is 2741... Forscan currently shows it set about 2651 (dont recall the exact number) . So, is my trip miles off causing the trip mpg reading to be off because of the tire size setting?

 

[WuNgUn]

I might have missed it in this thread, but I noticed the other day on waze, then on a gps app on my phone that my speedo said im going 2-3 mph slower than I was. This is on a sasquatch BD.

Is this a thing people are dealing with?

I think Ford purposely does this to the speedo, as do other manufacturers...
2 to 3 mph is a lot...
My Canadian Bronco is 2 Km/h off to the slow side.
This can't be just a variance in tire sizes.

 

[pmoore00]

In my recent experience, 2741 is WAAAAAY too big for the Sasquatch tires. I spent some time refining the number with both math and a GPS speedometer app and got to nearly perfect with 2579 as the circumference in millimeters for the take-off sasquatch wheels/tires I have on mine.

And… I went to the dealer to have the PCM relearn done.

 

[kput]

This happens on 2021+ F150's and 2020+ F250/350's as well. Not sure if it's affecting newer Mustangs yet.

FORscan is not yet able to correctly initiate the PCM relearn on the newer models for some reason. It works fine on the older ones. I know this is of no help since there isn't an older Bronco that the PCM relearn would have worked correctly on, but hopefully that provides some context to the situation. I suspect it will be resolved in the next several months with a new version of FORscan, but I don't have any inside track on FORscan development.

As posted above, the tool that the dealers use to do a PCM relearn does work, so that's an option. FORscan is free software that mimics $$$$ software/hardware the dealerships use, so that's why it's behind the curve a bit. Give it time.





While I am not going to say that Ford isn't being sloppy and getting things wrong, I can confidently say that there is a factor missing from the video in the OP and the instructions on the FORscan.org thread linked in the spreadsheet: a tire's revolutions per mile won't exactly equal the nominal diameter of a given tire size. You can confirm this for yourself by looking at specs on TireRack, which usually provides a revs per mile figure. Tires aren't rock-hard, they're inflated with air and have some give and squish to them. This is why the revs per mile figure differs from the nominal (or even actually measured) tire circumference.

Obviously, your speedometer needs calibrated to the revs per mile, not the nominal tire size.

In my calculator spreadsheet, I account for this by setting the final value to 96.7% of the calculated nominal size in millimeters. This is consistent with what Ford does for the F150, Mustang, Fusion, and a few other vehicles I've looked at. I've not poured over enough Bronco .ab files to see what they're using there. It's actually different for every tire model, but 96.7% will get you in the neighborhood. If you don't apply that correction factor, the truck will think it has a bigger tire than it really does and the speedometer will read fast.

My spreadsheet is here, you'll have to copy/paste the link to the Google Sheet since if I make the link hot, the forum will try to embed it and you won't be able to use it:

docs.google.com/spreadsheets/d/1mWnuQZlDTUGjKjmJje1QJIR8Z2c7Kf8KBZxKnw_8EqU

Make sure to follow the instructions at the top to make a copy that you can edit. You'll need to do this on a PC, it won't work on mobile.

If you want your speedometer to be dead-nuts on, there's a procedure at the bottom of the spreadsheet to do "GPS truing" as well. That's the best method if you're OCD about it. Every different model tire of a given size will have a slightly different revs per mile, which is where the variance comes from.



A word of warning about just leaving the P160A DTC in the PCM. That DTC is there for a reason and it's telling you the PCM is mad about something.

These vehicles rely on a number of sensors - four wheel speed sensors and a speed sensor in the transmission at least. Some Ford vehicles have a sensor in the diff too, not sure about the Bronco.

I can tell you from personal experience in a Mustang that after having corrected my tire size and then driving around with the P160A DTC, I experienced some drivability oddness that should give you pause. There was a dead spot at a certain RPM in a certain gear. When I hit that dead spot, I could floor the accelerator and the car would not speed up. Only way to get through it was to completely let off the accelerator for a beat and then mash it again.

Others (in the Mustang and F150 communities at least) have reported similar things. I think where it happens may be different for each tire size, because it's probably some mathematical formula that's freaking out once two values don't match by a certain amount. And that probably varies with how different your tire size is in your BCM vs what your PCM thinks it should be.

I'm not saying this could cause damage, it hasn't that I know of. But every Ford vehicle is a little different, and I'd hate to see a lean condition frying engines just because people want their speedometers reading correctly after fitting bigger tires.

These issues went away for me in my Mustang after doing the PCM relearn successfully.

So if I follow your spreadsheet and make the adjustment for tire size, what do I do with the previously set parameter that I inputted the MM circumference of my tire in to? Leave it where it is?



Also, for those having PCM relearn issuses, from what I've found is that it's an issue with the OBD device you are writing the changes with. My work uses a VCM 2 system and it was giving me the error at 2%. I ended up having to boot FDRS and run the TCM PCM relearns from in there.


My car failing the relearn procedure caused it to have a CEL involving the ABS system, as well as a Check 4x4 system light. If I didn't work at a Ford store and have FDRS access, I'd be rolling around with codes waiting for a new OBD adapter to show up in the mail.

 

[Spart]

So if I follow your spreadsheet and make the adjustment for tire size, what do I do with the previously set parameter that I inputted the MM circumference of my tire in to? Leave it where it is?

I don't follow. You can't put more than one tire size in your BCM.

 

[BroncoStile]

In my recent experience, 2741 is WAAAAAY too big for the Sasquatch tires. I spent some time refining the number with both math and a GPS speedometer app and got to nearly perfect with 2579 as the circumference in millimeters for the take-off sasquatch wheels/tires I have on mine.

And… I went to the dealer to have the PCM relearn done.

In my recent experience, 2741 is WAAAAAY too big for the Sasquatch tires. I spent some time refining the number with both math and a GPS speedometer app and got to nearly perfect with 2579 as the circumference in millimeters for the take-off sasquatch wheels/tires I have on mine.

And… I went to the dealer to have the PCM relearn done.

If you measure the height from the ground to the center of the hub it should give you the radius to use to calculate the correct circumference. In your case that measurement looks like it should be about 16.5”.

Its called rolling radius and it does vary based on air pressure and weight of the vehicle. Two different vehicles with the same tire can have a different rolling radius.

 

[pmoore00]

If you measure the height from the ground to the center of the hub it should give you the radius to use to calculate the correct circumference. In your case that measurement looks like it should be about 16.5”.

I actually did that. And used a string to measure actual circumference. Using the real world measurements the speedometer was faster than actual speed.

The most accurate approach I had was “my speedo says I’m going 70 but my gps app says 74… So I need to add 5.7% to the circumference in mm.” Spot on.

There is inaccuracy built into even the stock setup, so using physical measurements or tire size calculators is perfectly fine but the inaccuracy is baked in there too. Ignoring hard facts and using the relative method I used worked for me and the speedo is accurate at all speeds now.

 

[kput]

I don't follow. You can't put more than one tire size in your BCM.

Your spreadsheet has me putting xxxx 079D xxxx into the BCM. Will this overwrite the mm tire size I have already inputted following the ForScan instructions on adjusting tire size?


You mention a 96.7% adjustment. Couldn't I just load that adjusted figure based on your percentage into the tire size mm screen?

 

[Spart]

Your spreadsheet has me putting xxxx 079D xxxx into the BCM. Will this overwrite the mm tire size I have already inputted following the ForScan instructions on adjusting tire size?


You mention a 96.7% adjustment. Couldn't I just load that adjusted figure based on your percentage into the tire size mm screen?

That's exactly what you want. You can't have two tire sizes programmed into the BCM, there's only one spot for it.

By far the best way to get a dead-on speedometer is to use the GPS method though, not just the nominal tire size. Every SKU of a particular size of tire will likely vary by some percentage.

To use the GPS method, you need to know what value is currently in the BCM, what the indicated speed is, and what the actual GPS speed is. The instructions and calculator are on the bottom half of the Google Sheet I linked.

 

[LanCar]

Okay, got it now - I’ll keep monitoring the thread to see if someone else can figure it out. I would suspect the solution lies with FORSCAN. I am using the OHP SCANNER and they brag that it “works flawlessly with FORSCAN”! Is anyone pursuing this with FORSCAN - it has to be something related to their software…

Has anyone updated to Forscan 2.3.46 which was updated on 2/7 and tried the PCM relearn?

 

[wsdowns]

Has anyone updated to Forscan 2.3.46 which was updated on 2/7 and tried the PCM relearn?

I did the PCM "RELEARN" at the dealer at the first oil change - no more P160A error - everything is stable. The guy seemed knowledgeable enough, but I had to walk him through what I did. His comment to me was "okay, now FORD knows you have bigger tires" - yipee!

I don't think the PCM error would have been an issue for me - the tire size may be related to things I don't have or use. Example, the GPS, Advanced Cruise Control, etc. But, glad to see it gone. Getting rid of the AWD error and the wrench on the screen was most important.

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