Speedometer Forscan Fix for 37 Inch Tires

[LetEmRun]

It’s been awhile since I’ve done it, but if you go over a certain value on forscan in mm it will have a CEL that cannot be cleared with that value entered, I would have to get back into forscan to see what number I landed on but I was able to find a value that was slightly under the max # and very close vs GPS speed.

Cool. Would be nice if we all knew what the max value was that is allowed by the system. Could then work back from there

 

[TomSkerritt]

Cool. Would be nice if we all knew what the max value was that is allowed by the system. Could then work back from there

I want to say it’s 2850 or maybe even 2800 that you can’t go over. Mine come out to 2854 and that was throwing the cel so I decreased it until it wasn’t.

 

[LetEmRun]

I want to say it’s 2850 or maybe even 2800 that you can’t go over. Mine come out to 2854 and that was throwing the cel so I decreased it until it wasn’t.

Good to know, thanks!

 

[ZooDad]

Good to know, thanks!

Max value on newer trucks is 2770 give or take.

ZooDad

 

[LetEmRun]

Max value on newer trucks is 2770 give or take.

ZooDad

Thanks for this. I’ll plug in 2770 after my new tires go on and see what happens

 

[ZooDad]

Thanks for this. I’ll plug in 2770 after my new tires go on and see what happens

Use the tire size calculator to get you close

 

[LetEmRun]

Use the tire size calculator to get you close

I did reference your calculator. Converting to metric, my result was well above 2800. I am installing 37x12.5x17

 

[LetEmRun]

I did reference your calculator. Converting to metric, my result was well above 2800. I am installing 37x12.5x17

@ZooDad does this sound right?

 

[ZooDad]

@ZooDad does this sound right?

Yes. Start at 2770

 

[LetEmRun]

Yes. Start at 2770

Tried 2770 and 2740. Both threw codes that would not disappear even after relearn. Any thoughts?

 

[F450_on_44s]

I've programmed a ton of trucks and 2770 almost always works except for on some 2017 super duties. I would unhook the battery for 10 minutes to give it the ole reboot on the modules and try again. Are you changing the value in the body control module, then doing a pcm relearn and tcm relearn? You need to do that order for it work.

 

[LetEmRun]

I've programmed a ton of trucks and 2770 almost always works except for on some 2017 super duties. I would unhook the battery for 10 minutes to give it the ole reboot on the modules and try again. Are you changing the value in the body control module, then doing a pcm relearn and tcm relearn? You need to do that order for it work.

Ok. Yes changed in Body control module (plain English) then did a relearn. Tried clearing DTC and doing again. No luck.

 

[F450_on_44s]

Ok. Yes changed in Body control module (plain English) then did a relearn. Tried clearing DTC and doing again. No luck.

pull the battery cables for 10 minutes, I've had that solve a few gremlins. Especially since you just updated all of those modules recently. 2770 should work, but your speedo is still going to be off 2 or so mph with the 37's, so you may want to get an inline calibrator tool. I have the Hypertech one on my truck and was able to dial it in perfectly.

 

[ZooDad]

Also while working with ARod we realized the DTC we were still getting is the “Historical “ and not active. This clears with drive cycles
So a better question is do you have the wrench or CEL

ZooDad

 

[F450_on_44s]

the 2023 models freeze a DTC in the TCU. I've never checked in later to see if it clears over time but not within 50 miles of testing. You are 100% correct on some of them being historical and not current.

 

[BroncOh]

Also while working with ARod we realized the DTC we were still getting is the “Historical “ and not active. This clears with drive cycles
So a better question is do you have the wrench or CEL

ZooDad

Can you answer my question in the other Forscan thread? Thank you!

 

[LetEmRun]

pull the battery cables for 10 minutes, I've had that solve a few gremlins. Especially since you just updated all of those modules recently. 2770 should work, but your speedo is still going to be off 2 or so mph with the 37's, so you may want to get an inline calibrator tool. I have the Hypertech one on my truck and was able to dial it in perfectly.

Is it 2mph slower than actual after the change? I could probably live with that. Thanks for the info.

 

[LetEmRun]

Also while working with ARod we realized the DTC we were still getting is the “Historical “ and not active. This clears with drive cycles
So a better question is do you have the wrench or CEL

ZooDad

Interesting. Thanks.

 

[Alpoba]

Is it 2mph slower than actual after the change? I could probably live with that. Thanks for the info.

It 2 up to around in 50-60 and closer to 3 60 and up.

I had made the change on a 22 successfully by also making the change to axle ratio. I forgot the exact reason i did that, the logic was discussed somewhere on the forum. It went through without error codes, but my truck did not like that, something felt off and i reverted back for good.

Now on a 23 the change seems straightforward and almost everyone reports no issues making it. I decided not to do it for now, as you mentioned above i really don’t care for 2-3 miles off on speedo.

 

[Alpoba]

I had made the change on a 22 successfully by also making the change to axle ratio. I forgot the exact reason i did that, the logic was discussed somewhere on the forum. It went through without error codes, but my truck did not like that, something felt off and i reverted back for good.

Funny enough, it was this very thread where axle ratio change was discussed. Was a good lesson for me that not all forum advice should be taken seriously. I came back to warn people - do not mess with your axle ratio

Speedometer Forscan Fix for 37 Inch Tires

Edit: I can’t remember for sure if it allows you to use non-standard axle ratios. If it doesn’t, you’d need to move it down to the next lower standard ratio (from the example above it would be 3.55) and then move the tire circumference as well. (Standard ratio / calculated ratio) * tire...

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