This is the settings in the PCM on my stock 2014 tune. The first drop down box tells it whether to use the BCM tire size or the size in the pcm calibration. The Max. Tire Circ. is the max the PCM will accept without getting angry. 2532 corresponds to a ~33" tire, maybe a little less. My raptor 315/70R17's I set to 2672 after checking against my GPS.

So if you are trying to set the bcm to something >2532, thats probably why ur having an issue.

 

So I can select the 315/70/17 tire size in the drop down in the BCM. But if what you are saying is true, my PCM won't accept it because it is "out of acceptable range". Why would it be an option then?

Where did you pull the chart you have above (if not Forscan, what program)?

 

Yes, correct. I even tried doing that and got the code and incorrect speedometer previously. You have to get a tuner or one of the tire flashing devices to flash the PCM. SCT for example will change the PCM to the calibration value and ignore the bcm value. HPTuners can do the same but also allows you to increase the max allowable tire size so that the BCM value works.

That is HPTuners. Its tuning software.

 

I understand what HPTuners is. Thinking about the ranges you gave above for tire diameter and gear ratio I think I have plan.

I pulled out the old spreadsheet and did some calculating. My stock BCM tire circumference (taken from my original AS BUILT) is 2476mm as represented by 09AC on line 726-45-01. My stock tire size was 275/65/18. This size calculated to 2577mm. Dividing these two I came up with a correction factor of 0.9608.

Then I calculated my current tire (275/65/20) circumference and got 2737 and applied the correction factor to get 2630mm. Since this number is outside the range given I knew I would have to tweek gear ratio and tire size.

I divided 2630 by the current gear ratio of 3.55 to get 740.7682. I then multiplied this number by another gear ratio available (3.31) and came up with 2451.943mm. I will round this to 2452mm (0994 in Hex code).

So my plan tomorrow is change the gear ratio in the BCM to 3.31 and change the tire circumference to 2452 (0994 Hex code) on line 726-45-01.

My original line 726-45-01 is: 0163 - 09AC - 008C
This will get changed to : 014B - 0994 - 008C (these last four will be automatically recalculated when data saved)

I will report back tomorrow if this worked or not. It should as new gear ratio and tire circumference are both within the ranges the PCM accepts.

 

I dont think changing the gear ratio would change the speedo as its pulled from the wheel sensors.

 

So quick update. I entered the values I described above and did the PCM relearn function (top of the list of AS BUILTS in Forscan), reset DTCs (which cleared the PCM 0422 code). On my test drive my speedo and odometer were dead nuts on.

So from reading low by 6% - odometer would read 100 km though I traveled 106 km (speedo same error) to being bang on over a 25km test drive.

For the speedo, my eyes could not discern a difference. I set my cruise control to what looked like exactly 120 km/h and within a couple seconds the GPS speedo settled on 120. I adjusted speed over the 60 to 120 range and each time the GPS settled on exactly the speed I had on my speedo.

Thank you mass-hole for the insight into the limits of what the PCM will accept from the BCM for values for gear ratio and tire diameter. Now I can calculate the correct values for any tire diameter/gear ratio combo and should get the speedo correct.

Conclusion ... success.

 

This is the second thread where I've seen you make this claim, and your claim is simply false.

It is indeed possible to get your speedometer readout to be well within ONE MPH of a GPS speed display.

In my case it is using a Superchips Flash Cal(ibrator) and a vehicle GPS app named "Speedometer" on an iPhone 10.

For example, the FlashCal uses .25" increments for tire diameter input, but for best accuracy you must use the tire's loaded radius. In some cases, depending on how lightly or how heavy your rear axle is loaded and your measured tire Cold Inflation Pressure, the tire's loaded radius can be 3/4" shorter than what is 1/2 of it's overall diameter. One measures this on level ground. I measured at the right rear tire. It is more precise if you can remove any wheel centercap to find the exact axle center.

This may be not usable in FORScan if FORScan uses metric tire circumference data, but the point is that regardless of what software interface one is using, getting speedo calibration results to within ONE MPH is achievable if your input data is accurate.

Three MPH difference is unacceptable.

 

Forscan can adjust the speedo within 1mph - at least on the 2011-14 models, maybe more. I have done it.

Here is how I did it...
https://www.f150forum.com/f38/speedo...ration-555868/

 

Kruiser! I think I need your help... I have a question about how FORScan works relative to an SCT FlashCal.

I have been using a SCT FlashCal to adjust tire size over the last two sets of tires. I input what the loaded radius is (16.00") X 2 to equal the tire diameter that the axle sees (or according to mass-hole, what the wheel sensors read). (for the sake of any FORScan data entry, this would be 2553 mm).

My axle is a 3.31. The SCT FlashCal accepts gear ratio selections up to 3.73... but next week I will be having installed 4.10s.

I am concerned that any inputs I've written to the truck from the FlashCal may obscure what I now find I must do through the FORScan interface...

[Edit: I see your link in the preceding post - I'll start there. Thanks!]

 

Is SCT FlashCal for speedo correction? Or is it a tuner as well. If for speedo correction only, I would think you would not need it at all if you adjust parameters in the BCM using Forscan.

Now what I have been doing is on the '11-14 models, so I can not guarantee it will work on the newer models. From your sig I see you have a 2016. I think it will still work, but the line to change is a different number. It looks like the line for the 2015-19 models is 726-12-01. It appears to lay out the same with the hexadecimal value for the gear ratio as the first 4 characters and the tire size as the next 4 characters. Try coming up with new numbers to enter using the linked thread above, if not post your AS BUILT line 726-12-01 along with the new tire size and axle ratio you either have or are going to and we can figure out some values to try ( I say try because I have not applied my theory/calculations to a '15 or newer).

Let us know how it goes.