KRUISR, you are correct. The As Built data as read yesterday is not the same as the As Built data from the Ford Service link you posted.

Here's what FORD sees as As Built for my truck's VIN -

726-12-01 . . . 014B 0932 561D

 

So that means the factory tire circumference value is only 32mm bigger than what I estimated/guessed yesterday. I will plug this info into my calculator and give you a different gear ratio and tire circumference to plug in with forscan. If it shows virtually no change in your current speedo error value then it confirms we can make the correct adjustments for your new gears.

I will post something later this afternoon with a new line 726-12-01 to try.

 

Okay. Try this in line 726-12-01 0140 098F xxxx

This should not change your speedo when compared to the GPS speedo (let me know if it is any more off than you currently have). Change it in your BCM with Forscan and do the PCM Relearn and clear the codes. Take it for a drive and see what we have.

What I am beginning to figure out is that FlashCal simply changes the same line in the BCM (based on the changed values from factory number in your As BUILTs.) and does the PCM relearn, just without Forscan or other scan tool.

 

10-4. So that we're only making one change at a time (so to speak), I am not going to try to do ANYTHING right now with the FlashCal. Not going to touch it. I have not touched it for anything recently except when I hooked it up last week to read only - to read only what axle gearing options it gives.

The SCT FlashCal options stop at 3.73 (from 3.15). It is, however, the way I achieved my synched speedo to GPS six months ago when I put these latest tires on.

I will only use FORScan, and only change the line 726-12-01 to the values you've just posted. PCM relearn, but I'd rather not clear any DTCs just yet unless you think a DTC would interfere w/ the speedo calibration you are helping me with.

 

You need to clear the DTCs because when you change the BCM and haven't done the Re-learn it could throw a code that the PCM and BCM don't match. So clearing them once both tasks are done will make sure that everything is clear without any old codes. Any other issues on the truck will still generate codes again, but at the start up moment after these changes, no codes should be present.

Yes, the change I am asking is to verify the theory I want to apply to get you sorted out after the gear change. There are many combinations of gear ratio and tire size that will keep your speedo correct. This exercise is just checking my understanding of how your truck is today. If I am right, I can have you a new line 726-12-01 in about 2 minutes or less.

 

Well, you're at 99%!

I entered the data you suggested 726-12-01 - 0410 098F (and got a new cheksum xxxx). I did not see your latest post so did not clear any DTCs yet.

But when on the test drive and as was before, with my analog-looking dial speedometer steadily positioned right on the money at 50 MPH [and also as before, my (Cruise-)Speed Control setting read 49 - 1 MPH difference) the GPS Speedometer read 51 MPH.

When Writing the data and executing the FORScan prompts, the dash chimes went off hi-lo hi-lo hi-lo as they would do when I had done (in the past) tire size using the FlashCal.

The only difference after turning the Iginition Off then back On was that my tire pressure monitor readout for all four tires (my default, tiny info screen display) displayed no pressures for about four blocks into my drive. Once out on the parkway (a three-mile-long stretch with few traffic signals) was where I got the first batch of MPH readouts.

You're very close! Thank you again! for your efforts. And while I certainly would like to be right. on. the. money, I'd rather my dial speedo read 1 MPH fast than 1 MPH slow relative to GPS for obvious reasons.

 

Excellent, glad it is working out.

For the next week, if you want to make it more accurate change the tire circumference from 098F to 09BC. It should put your dash speedo maybe 0.5 MPH higher than the GPS and your cruise about 0.5 MPH lower. See how close I am.

If that makes it closer to what you just tested, then here is your line 726-12-01 after your gear change...
726-12-01 0177 092C xxxx.

That should be good for your existing tires and the 4.10 gears. If your speedo is reading lower than the GPS, you could increase the 092C to 093B for a 0.5 MPH change or to 094A for a 1 MPH change. Conversely if your speedo is reading higher than the GPS you could decrease the 092C to 091D for a 0.5 MPH change or to 090C for a 1 MPH change.

We can change the tire circumference in 1mm increments. I have calculated elsewhere that about 30mm of a change in tire circumference value should be about 1 MPH change to the speedo relative a GPS speedo - up or down from a given point. That means it is theoretically possible to dial in a speedo in 1/30th (0.0333) of a MPH increments - if you are so OCD of such a thing.

Don't forget as your tires wear the speedo will change as the diameter decreases - not likely in a significant way that most would EVERY notice while driving.

Let us know how it goes.

 

Dude...

 

I can do this tomorrow (Saturday) morning and report back in the afterno0n.





This ^ is the data I will enter before leaving the gear shop. I will post again when I get home much later in the day. The drive *would be* 45 mins at interstate highway or "freeway" speeds, but since I'll be breaking in a new ring-and-pinion, I will be using surface streets only to travel the 40-odd miles back home. I expect this to become a major voyage, stopping occasionally to let the gear set cool, also stopping for a meal, etc... because the first 100 miles of the 500-mile break in process is critical to gear set longevity.





Except me. lmao. Kidding. I know what you're saying, though.





I certainly will, and with our results played out with my truck, perhaps this info may be useful to anyone else doing an F150 gear swap in the future. You rock!

Too, I want to thank in advance F150forum users 53 Merc, PacificTrekker and a thread starter on the topic of mild overlanding rigs, forum user Shmee. All have been helpful in my endeavor, but it is KRUISR and mass-hole in this very thread started by GrizzHair700 just over 1.5 years ago that have put the icing on the cake. User RoadPilot, too.

To all of these fellers:

 

Saturday morning...


Have input 09BC but it wasn't quite enough of a change:

May I please buy another 1 MPH?

Speedo is now damn close at 50 but still 1 MPH slow reading 70 to the GPS' 71 MPH.

So, before the gear swap in a few more days, we need a slightly less mm hexidecimal for tire circumference, or however you're mixing it up w/ gearing.