I’m looking for someone to clearly explain how they recalibrated their speedometer.

2013 F150 FX4
35 x 12.50 x 20 tires

Things I’ve done with no luck:
- Forscan module update typing in circumference in millimeters and saving it, then relearning PCM, and clearing DTC codes all with Forscan
- Tried to use hex keys with online calculator and relearn PCM and reset DTC codes

Both of these two processes did not work for me. I’m still 4mph off at 70mph (speedo reads 66, GPS says 70)

Can anyone explain what they did to get theirs to calibrate using Forscan? I’ve searched high and low on the forums, but no additional luck.

*Also, cannot get U0422 code to reset… maybe this has something to do with it…

 

I can walk you through what needs to be done. A couple questions first...

Using Forscan, what is line 726-45-01 in your BCM AS-BUILT file (hopefully you saved your factory data)? What was your stock tire size?

The PCM has limits that it will accept for Tire Circumference and Gear Ratio. A 35" tire are about 110" circumference which is 2793mm (true circumference will be less and is effected by air pressure, tire wear and manufacturer). If you can find the manufacturers declared tire diameter or circumference, it will help dialing in the best settings. Out of curiosity, what have you been using for tire circumference (in mm)?

A fellow member here on the forum (mass-hole) enlightened me as to the limits of the PCM which was the "light bulb" I needed to get the settings right.

The circumference range the PCM will accept is 2296 - 2532 mm. Anything entered larger or smaller will be ignored and the PCM will return to the factory default values when you do the relearn.

If I remember right, Code 0422 will go away when the BCM and PCM have the same info after the relearn.

Anyway, post up the info I asked above and I will walk you and everyone else through the calculations to dial in your speedo (at least in a 11-14 model). Then if you change tires in the future you can recompute yourself.

 

Make sure your speedometer is sitting at zero when the truck is turned off. These truck have a tendency for the needle to slowly creep away from zero at rest.

 

Thanks a ton!

 

All right, so with your stock tire size (275/55/20) it calculates out to:
(275 x 0.55 / 12.5) + 20 = 32.1" in diameter. Multiply by pi which calculates to 100.84" circumference. Multiply this by 25.4 to get circumference in mm which equals 2561.

The factory adds a correction factor to account for tire squat and other factors as well.

Why did I ask for line 726-45-01 info. Because it has your corrected tire circumference and the gear ratio. The first 4 characters represent the gear ratio in HEXADECIMAL. In your case 0163 calculates to 355, so 3.55 rear gears. The next 4 characters are the corrected tire circumference. 099F calculates to 2463.

So now we need that correction factor the factory put in. We divide the corrected circumference by the calculated circumference:
2463 / 2561 = 0.9617

Now on to your new tires: 35x12.5x20. So we redo the calculated circumference with the new tire diameter (tire manufacture declared diameter would be more accurate if you have it):
35 x pi = 109.95" and then multiply by 25.4 to get 2792.87 mm. We round that to 2793 mm. We then apply the correction factor from above: 2793 x 0.9617 = 2686 mm (rounded).

Given the accepted tire circumference range I listed in a previous post (2296 - 2532) you can see that your corrected value would be outside this range. Since the truck PCM uses tire circumference, gear ratio and rpm to figure out your speed to display on the speedometer we just need a different combination of tire circumference and gear ratio that will represent the same thing as your actual values (basically fake out the computer - but it works with no side effects that I have noticed).

Just like tire circumference, there is an accepted range for gear ratios as well: 3.13 to 4.12 Any values outside this range will be ignored by the PCM.

Next we take your new tire size corrected circumference: 2686 mm. We divide it by your current rear gear ratio : 3.55.

2686 / 3.55 = 756.6197

Now we pick another gear ratio that will be smaller than your current ratio (because your tire circumference is higher than the accepted range) but still within the accepted range. In my case I used 3.31 - mainly because I felt like it and it is an existing factory gear ratio. In this case, having played with the numbers a bit, lets use 3.20 for a new ratio. We multiply the above by this new gear ratio and get...

756.6197 x 3.20 = 2421.18 and we round to 2421 This is within the accepted range so we can use it. If it wasn't we could go back and change to 3.20 to a smaller value (still within the accepted gear ratio range) and recalculate this last calculation. But in this we can use 2421.

Now we have a new tire circumference and gear ratio and need to convert to hexidecimal. I use the following formula in an EXCEL spreadsheet to convert:

=DEC2HEX(xxxx) where xxxx is the value you want to convert.

Using the formula 2421 converts to 975
320 (drop the decimal in the gear ratio) converts to 140

This makes your new line 726-45-01: 0140 0975 007F (don't worry about the last 4 digits, these are a checksum number and the computer will recalculate these when you save the data to the BCM)

Save your AS BUILT data and do the PCM Relearn function. Clear your DTCs and when you re-scan for codes the 0422 code should go away.

Go for a drive and see how close your speedo is to a GPS speedo.

If the GPS speedo is high and you want to make it more accurate, go back and increase the 2421 a few mm. Playing with some calculations I came up with a general increment of a 30mm change in the 2421 number will effect the speedo by about 1 mph (up or down).
So if your speedo is reading 60 mph and the GPS speedo is at 61mph - you will want to increase the tire circumference, you could change the 2421 to 2451, convert this to Hexidecimal, change your line 726-45-01, do the relearn and you should be bang on. Vise versa if your speedo said 60 mph and GPS said 59 mph and you wanted to correct it, you would decrease the 2421 number to 2391 and you should be bang on. Keep in mind you still need to be within the accepted ranges for tire circumference and gear ratio or it won't work.

In conclusion - with 35x12.5x20 tires with a 3.55 gear ratio in the truck, a new line 726-45-01 as follows should correct your speedo.
726-45-01: 0140 0975 007F

One more thing, speedo accuracy will change over time as your tires wear or your replace them again (especially if with a different brand).

I know this reply is long winded but I wanted to explain it to a level that many will be able to do this adjustment on their own and as often as they need to.

Let us know how it goes.

 

This worked perfectly!

Thank you for the clear, concise instructions and explanations. Hopefully this will help many others!

topic closed.

 

I took the calculations above and created a spreadsheet to simplify things.

You will need your factory tire size, line 726-45-01 from the BCM and what tire size you are putting on or have put on.

Hope it helps.

 

Wow! Kruisr, this is great work. Thanks so much. I do wonder if you could 'dumb it down' a bit more for me. I have stock tires and 3.73 gears. I'm not looking to change anything except to correctly calibrate my speedometer. I'm new to F150 crew and new to Forscan so I'm working to 'catch-up'.

Is it posible to change the Tire Circumference in the BCM ONLY? Or do I have to go into the As Built section as well? I have assumed that they are duplicate of each other and the non AS Built version is just simpler for non programming folks like myself.

I've downloaded your spreadsheet and it's not clear to me how to use it. Like I said, I am not changing tire size, so I don't think I need to fake out the ECU with a fake gear ratio.

I appreciate your time.

 

If tire size and rear end gear are still stock, you should be able to tweak the non AS BUILT table to tune in your speedo. Is your speedo currently reading high or low of actual GPS measured speed? What is the current Tire Circumference number in the BCM? What is your current tire size?

 

Thanks. I"m back to only 5% off. I'll take it given my 10% off last week.

This weekend was troubleshooting a bad coil - that's replaces, so now I'm back to 'better than I was before'.

I appreciate the help

Aaron