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3.73 to 4.88 Gear Swap Problems in 2020 Bullitt

GreenS550

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So you're saying that they changed the tire sizes you can enter for 19 and up? Remember I'm not referring to the code for changing the actual ratio.
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K4fxd

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Here is the allowed limits on this tune. I allowed a 5.13 max gear

range.png
 

GreenS550

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That's not where you change the gear ratio in Forscan.

Check this link out made by a guy that did it primarily for guys putting giant tires on their trucks. If you do the math on revs/mile and then work backwards you can make it happen.

 

K4fxd

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Forscan will not write to the PCM and the limits are in the PCM

In the tune I posted the max tire circumference is 2201 mm, min is 2019. If you enter 2300 mm in forscan the PCM will not recognize it
 

GreenS550

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Schwerin

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But does that affect both the speedo and Odometer? If not, then it's pointless.

I'd also rather not use a weird hack/work around to get something like gearing ratio working properly.
 

K4fxd

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The BCM sends the info to the PCM if the BCM's info is out of range it won't see it and it throws all sorts of lights.

If the PCM is programmed to see 700 to 800 revs per mile and you tell the BCM to send 900 revs to the PCM it freaks out.

Don't take my word for it, try it.

Also search around the forum and you will find several who have tried this and needed a tune.

This only happens if you actually put the tire or gear in the car. If you just change the BCM settings but do not change parts the PCM ignores your bogus input.
 

GreenS550

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You don't change the revs per mile! You change the code for the tire size.
 

GreenS550

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PCM does not have anything to do with it. You're not changing the gear ratio! Go back and reread the post. I just did this help others if you don't want to do it that's fine.
 

K4fxd

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I showed you what is in the tune. If the BCM and sensor data are out of range it lights up the wrench light.

People have tried it and the only way it works is with a tune.
 

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GreenS550

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I showed you what is in the tune. If the BCM and sensor data are out of range it lights up the wrench light.

People have tried it and the only way it works is with a tune.
Well maybe that was the case for you but it wasn't for me. When I change the BCM to the different gear ratio no lights went on there were no issues except the car would go in limp mode over a certain RPMs. But when you change the code for the tire which you have to use that spreadsheet I posted and if you haven't done that then you don't know. It sounds like you're assuming something that you haven't actually done yourself. And need to click that link that does the tire calculation and copy it and then paste in the tire size. You use the code that it reveals into the BCM.

If you're trying to argue you're wasting your time but if you really want to know try it yourself when you change your axle ratios. Another fallacy on the forums is that you can change the axle ratio and that since it picks up the speed from the ABS sensors you don't need to reprogram the computer and that's wrong. But this is a workaround which fools the car into thinking that the axle ratio has stayed the same but because the tire diameter is different it has to calculate that out for how the car runs. And it works!

if you really spend some time searching this on the web you'll find a number of people have done this I'm not just Mustangs but other Fords.

And for those that don't want to do it there's always the option of a custom tune which will probably make more power anyhow.
 

K4fxd

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This is from ForScan themselves and mirrors what I said. If out of range it will not work.


Note: some modules have a harcoded range (min…max) for the tire size. So if new value is out of the range, module will not learn it but throw P1635, U0422 or similar DTC. Unortunately, in such cases we have no choice other than adjust the new size to be in the range (so, find maxmum circumference accepted by the module).
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