Correct your speedometer error - no flash or tune required

Spart

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So we've finally figured out how to correct speedometer error (if your car is just off from the factory or you've changed tire sizes.) Thanks to @solodogg for figuring out where in the as-built data this info might be stored, even though I'm sure he'll give me all the credit for decoding the way Ford programs tire size in hex.

What do I need?

To do this you will need a laptop running Windows, the free FORScan software with a free extended license, and some sort of ODB reader that works with FORScan. I recommend and use the ODBLink MX: https://www.amazon.com/dp/B006NZTZLQ or the ODBLink EX: https://www.amazon.com/dp/B081VQVD3F If you use a Bluetooth reader like the ODBLink MX, you'll need to make sure your laptop has Bluetooth or else you'll need to buy a cheap USB Bluetooth dongle.

This is not meant to be a detailed guide on modifying as-built data, there's a lot you can do with it - some that we've figured out and probably more hiding away if you want to try your sleuthing skills. Check out this thread for a complete guide on how to use this software, including how to download it and get an extended license: http://www.mustang6g.com/forums/showthread.php?t=61773

Also check out @fionic 's S550 As-Built spreadsheet for other known working mods, like changing your rear-end gear setting, removing the double-honk or enabling fog lights with the high beams on: click here



How does it work?

For 2015-2019 Mustangs, Ford programs the tire size of the car on BCM line 726-12-01 in the second four-digit block of hex. For 2020+ Mustangs, it's split between 726-15-01 and 726-15-02 as described below. It's stored as the nominal circumference of the driven wheel in millimeters with a revs/mile conversion factor of 96.7%. To save you the trouble of doing a bunch of math and conversions between decimal and hexadecimal, I've come up with a hex calculator that you can use here: click here

This is covered in the spreadsheet, but for example my 2017 GT PP currently has this in the BCM as-built:

726-12-01 xxxx 0858 xxxx

If I changed my rear tires to 305/35R19's, my calculated value is this:

726-12-01 xxxx 0843 xxxx

The x's mean to leave whatever value is already there.

2015-2019 Mustangs share the 726-12-01 address. For 2020+ Mustangs, the location has changed to this:

726-15-01 xxxx xxxx ABxx
726-15-02 CDxx xxxx xxxx

ABCD = your calculated value. It's split between the two addresses for 2020+ cars.

You also have two options here. You can simply enter the size of your current rear tires as in my example above, and using the nominal diameter of that tire size the calculator will give you a hex value you can program to get pretty close (maybe even dead on) with the speedometer. Tire manufacturers vary of course, and your car's speedometer may be a little weird. So below the tire size calculator is a truing calculator that relies on a precise measurement from GPS.

As stated in the calculator, I recommend that you use a GPS that can output in tenths of a mile per hour, that you set the cruise on level ground as high as reasonable (eg 75mph) and that you use the needle rather than the digital speedo or digital cruise setting to set your speed, since the digital outputs are rounded to the nearest whole MPH. That is to say, you might see that the car is set to 75mph, but internally the car could be set to 74.5 or 75.4, and you'd never know because it's rounded before it's shown to you. Here is an example screen showing tenths of a MPH from a Garmin on the satellite info page (press and hold on the satellite reception strength bars in the upper left corner to get to this page.)

garmin-satellite-info.jpg


Try to set the needle exactly on one of the MPH hash marks on your speedometer for best results. Move your head to the right to eliminate parallax error at that speed, see the below video.

FWIW, if you have other Ford vehicles this seems to work on them as well. I've personally confirmed that it works on F150s (thanks to Freedom Racing for letting me borrow their shop truck) so if you've got a giant bro-dozer with 35" tires, it's time to fix that speedometer!



I have a DTC now.

You WILL get a non-MIL DTC P160A (Control Module Vehicle Options Reconfiguration Error) when you change this in the PCM.

dtc-pcm-after-speedo-adjust.jpg


Thanks to the FORScan devs and some beta testing adventurers, we are able to reliably remove this DTC now using a new feature.

This new feature is only available in FORScan 2.3.11 or higher.

What you want to do is go into the computer-chip-looking tab in FORScan where you normally edit as-built. At the top, you'll see a new function for the PCM - "Module initialization/relearn vehicle data" - see this screen shot:

forscan-pcm-relearn.jpg


Highlight that function and hit the play button, and follow the onscreen instructions to the letter.

Once it's finished, rescan for DTC's and remove the P160A DTC. Go drive around and scan for DTC's again. You should be in the clear.

Note that if you ever wish to return to stock, you'll have to run this procedure again after setting the as-built values for the tire size back to stock.



What should I do if I haven't changed my tires but my speedometer doesn't match my cruise setting? The speedo reads fast!

I feel this needs to be addressed, because people don't seem to believe it.

There is a *large* amount of parallax error in these cars when the needle is pointed up, which it is when you're at highway speeds of around 70-80. At those speeds your speedometer will appear to read 2-3mph faster than your cruise setting.

Why? The needle is thick and spaced out from the gauge face, so the perceived "needle" (the illuminated red center section) will give a great deal of parallax error unless you're viewing perfectly perpendicular to the needle.

Unfortunately, the designers chose to put the speedometer in such a position that reading the speedo with your head perfectly perpendicular to the needle means moving your head WAY right, far away from any natural position in the car.

All that academic bulls--t people probably won't read, so here's a video that illustrates the effect:

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Need4SpeedMotors

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[MENTION=11685]Need4SpeedMotors[/MENTION], I'm turning into a skeleton over here waiting for your GTPP/GT350 spare tire solution!
lol I come on man im trying over here.
 

lwnslw

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Can this be done using an SCT X4 tuner ?

Since I installed my 20's, my mileage is off. My office is 18.3 miles away, since I installed the 20's it now reads 19.5. As you can see, Im adding uneeded mileage to my Stang.

Thanks
 
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Spart

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Probably not using this method, you need something that can work with FORScan so you can modify the as-built.

Tuners modify the firmware, the as-built is like the settings for the firmware. Two different areas. Changing the settings doesn't alter your flash count either.
 

Jmeo

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Wow, this is just amazing! Thank you for your time and effort in this huge accomplishment. I have the OBDLink and FORscan already so now Ill just pop in and adjust.

Thank you!!

:cheers:
 

dubster99

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Sorry if this is covered in another thread, but what are the potential risks of using this to get in and modify this code?
 

solodogg

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Sorry if this is covered in another thread, but what are the potential risks of using this to get in and modify this code?
Really, there aren't many risks involved, but it depends on your comfort level with making programming changes. In most cases if you accidentally try to write an incorrect value, forscan will tell you it's incorrect. If you somehow write an incorrect value, but with the correct checksum for the last bit, you will have to re-write the original code back to the car.

Your entire as-built can be found at http://www.motorcraftservice.com/AsBuilt so you can refer back if something gets changed by mistake.

With all of that said, if you're not comfortable with simple programming, I wouldn't recommend taking this task on yourself. While as-built files can be corrected, an incorrect value could possibly cause issues if you don't know how to restore the original. Spend some time reading the as-built thread over in the DIY forum, and read through the forums at forscan.org before deciding if you are comfortable making these changes on your own.
 

dubster99

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Thanks for the heads up. Doesn't seem too tough as long as you locate the correct code line...shouldn't be too difficult. I really only want to change the speedo for tire size.
 

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Using your calculator the value I get for my 325/30/19 tires is xxxx 080A xx

The line in BdyCM is [726-12-01] 013B 0827 5500

I thought the value from the calculator would go in the second block "0827" but as you can see I have all numbers and the calcualtor gave me 080A is this correct?

Also the calculator gave me xxxx 080A xx
the line I have is 013B 0827 5500 which is 2 more places than the line from calculator
 
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Spart

Spart

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Using your calculator the value I get for my 325/30/19 tires is xxxx 080A xx

The line in BdyCM is [726-12-01] 013B 0827 5500

I thought the value from the calculator would go in the second block "0827" but as you can see I have all numbers and the calcualtor gave me 080A is this correct?

Also the calculator gave me xxxx 080A xx
the line I have is 013B 0827 5500 which is 2 more places than the line from calculator
You want to change digits 4-8 (change 0827 to 080A.) The last two digits are check digits that FORScan takes care of automatically.
 

Jmeo

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You want to change digits 4-8 (change 0827 to 080A.) The last two digits are check digits that FORScan takes care of automatically.
OK thank you! When I saw the letter in there I thought it needed to be all numbers :frusty:
 
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Spart

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OK thank you! When I saw the letter in there I thought it needed to be all numbers :frusty:
Nope, as-built data is in hexadecimal which is represented as numbers 0-9 and letters A-F.
 

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Nope, as-built data is in hexadecimal which is represented as numbers 0-9 and letters A-F.
Sure hate to keep bothering you but here is the before and after and I was just wondering if you think this looks right?

before: [726-12-01] 013B-0827-5500
after: [726-12-01] 013B-080A-55E3


I also saw some trouble codes while in there:







These were there before I did anything. Don't know whats up but the parking lights work just fine :shrug:

The only thing I can think of is maybe when I was installing my Roush active exhaust I connected the battery at one point while the taillights were still out? This has to be what caused the codes to show up. If I interpret them correctly its saying they were there at one point but are not currently an issue?
 
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