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Recalibrating speedometer with tire change

Red_shift

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Soon to be new Jeep owner here with a '22 Willys on order. I want to swap out the stock 255/75r17 MT's for 285/70r17 AT's. Is it necessary to buy something like an AEV Procal SNAP to recalibrate the speedometer? Any drawbacks to just leaving everything as is after the tire change?
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Slaw32

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The dealer can do it and you won't have to be concerned going forward when you bring it it in for service . Contact the service manager , and or your salesperson and negotiate it in advance of the arrival of your jeep . If you let them know in advance and they may do it for no charge .
 

Rhinebeck01

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@Red_shift

1st off, I encourage you to not even consider installing an AEV ProCal Snap.

AEV is a very decent company but the Snap is to be avoided... Big issue is with the connector pins that the Snap installs with ... with them coming loose/breaking etc..

For tire size recalibration, just install the Smartphone app, "JSCAN" , register your JL's Vin# with JCAN ($22 approx.) and buy the needed, inexpensive Bluetooth OBDll Scanner and a bypass module. Cost for all will be about $75.00. To do the tire calibration with the aforementioned takes but a few minutes to do and is easy to do...

By the way, JSCAN offers other settings... like fogs on with high beams and more...

.
 
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Slaw32

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@Red_shift

1st off, I encourage you to not even consider installing an AEV ProCal Snap.

AEV is a very decent company but the Snap is to be avoided... Big issue is with the connector pins that the Snap installs with ... with them coming loose/breaking etc..

For tire size recalibration, just install the Smartphone app, "JSCAN" , register your JL's Vin# with JCAN ($22 approx.) and buy the needed, inexpensive Bluetooth OBDll Scanner and a bypass module. Cost for all will be about $75.00. To do the tire calibration with the aforementioned takes but a few minutes to do and is easy to do...

YES, if you have an automatic tranny JL, you will want to recalibrate for tire size.. Wrong tire size means your automatic shift points will be off.

By the way, JSCAN offers other settings... like fogs on with high beams and more...

.
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Have the dealer do it , for no charge .
 

Strommen95

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Soon to be new Jeep owner here with a '22 Willys on order. I want to swap out the stock 255/75r17 MT's for 285/70r17 AT's. Is it necessary to buy something like an AEV Procal SNAP to recalibrate the speedometer? Any drawbacks to just leaving everything as is after the tire change?
Your speedo will be slightly off, very slightly. That's it. Shift points are not affected. Whether it's necessary or not is your discretion.
 

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Rhinebeck01

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@Slaw32

Reality is even the dealer pays the manufacturer when they use the WiTech ll to calibrate tires or do other deeds that require the use of the WiTech ll.

Sure you may be able to get your dealer to do your tire recalibration but chances are very very slim, (possible they might) that they will do for free...

No harm asking your dealer but again, highly unlikey that tey will do for free.

.
 

Slaw32

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your dealer may do it for no charge . You are spending over 50,000 . If you ask before taking delivery they will probably do it for no charge , or you can pay for it , your choice
 

TX_Ovrlnd

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@Strommen95

Shift points are in fact impacted / will be off ..... ..... especially noticeable when bigger tires are in the mix. OP will see little impact with the tires he is moving to..
They're only altering the tire size, it does not change transmission shift points in the TCM. If going to a very large tire without changing gears then it will seem to hunt gears more, but it's not altering the tuning.
 

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Not to threadjack but I asked the same in another thread. Getting a 4XE and thinking of upgrading from the stock 275/55 to 275/60. Not much of a difference at all but should I worry about shift points/recalibrating the speedo for such a minor upsize?
 

Strommen95

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@Strommen95

Shift points are in fact impacted / will be off if speedo is not calibrated ..... ..... especially noticeable when bigger tires are in the mix. OP will see little impact with the tires he is moving to..
No, not accurate. The transmission's input for speed is from the vacuum modulator. That's mechanical fact. Not the ECU. The speedometer or tire size in the ECU is not relevant to shift logic.
 

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Rhinebeck01

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No, not accurate. The transmission's input for speed is from the vacuum modulator. That's mechanical fact. Not the ECU. The speedometer or tire size in the ECU is not relevant to shift logic.
Not worth going around on this.. but.... while the shift points aren't technically adjusted lets say, it is just a matter of the shift points being based, in part, on the vehicle speed. If the vehicle speed is not correct, that can have a negative effect on shifting. This has been discussed many times on this forum in regard to the JL.

Again, little change in tire size from stock, not that big a deal to recalibrate but...
 

TX_Ovrlnd

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Not worth going around on this.. but.... while the shift points aren't technically adjusted lets say, it is just a matter of the shift points being based, in part, on the vehicle speed. If the vehicle speed is not correct, that can have a negative effect on shifting. This has been discussed many times on this forum in regard to the JL.

Again, little change in tire size from stock, not that big a deal to recalibrate but...
Yet here you are, I'm just trying to prevent misinformation. Tire size calibration corrects the ecm's calculations of revolutions per minute of the tire so it can correct what is shown on the speedometer. The negative impact on shifting is from not regearing to account for added tire size, it's causing the engine to work harder thus not being able to hold at the higher gear (7 or 8). This is the gear hunting.
To alter shift points you will have to reprogram the ecu and tcm, I did it with BlackBear in my old chevy vortecmax, it was amazing.
 

Strommen95

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Not worth going around on this.. but.... while the shift points aren't technically adjusted lets say, it is just a matter of the shift points being based, in part, on the vehicle speed. If the vehicle speed is not correct, that can have a negative effect on shifting. This has been discussed many times on this forum in regard to the JL.

Again, little change in tire size from stock, not that big a deal to recalibrate but...
My comment already answered your falsehood. The transmission reads speed manually through the vacuum modulator. It doesn't matter if the ECU's tire calibration/speedometer is dead wrong, the input isn't relevant to the transmission.

It's been discussed many times. Many times by people who were ill-informed and then the next poster is ill-informed too. It creates an echo chamber of bad information being regurgitated.

Gears Magazine - The ZF 8HP: It's Shift Strategy, No Problems Here! Adaptive Strategies and How They Affect Your Diagnostic Process

Nothing on speed or tire size.

Tazer vs Flashcal vs other JL programmers | Page 16 | Jeep Wrangler Forums (JL / JLU) - Rubicon, Sahara, Sport, 4xe, 392 - JLwranglerforums.com

The man who has a financial interest in telling you shift points would be off says they aren't.

1c02e071-af9f-45b7-bfe1-d3abb19d4a90-jpeg.455259 (1900Ă—1425) (jlwranglerforums.com)

Funny how the manual doesn't list "speed" or "tire size" as an input.
 

Rhinebeck01

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My comment already answered your falsehood. The transmission reads speed manually through the vacuum modulator. It doesn't matter if the ECU's tire calibration/speedometer is dead wrong, the input isn't relevant to the transmission.

It's been discussed many times. Many times by people who were ill-informed and then the next poster is ill-informed too. It creates an echo chamber of bad information being regurgitated.

Gears Magazine - The ZF 8HP: It's Shift Strategy, No Problems Here! Adaptive Strategies and How They Affect Your Diagnostic Process

Nothing on speed or tire size.

Tazer vs Flashcal vs other JL programmers | Page 16 | Jeep Wrangler Forums (JL / JLU) - Rubicon, Sahara, Sport, 4xe, 392 - JLwranglerforums.com

The man who has a financial interest in telling you shift points would be off says they aren't.

1c02e071-af9f-45b7-bfe1-d3abb19d4a90-jpeg.455259 (1900Ă—1425) (jlwranglerforums.com)

Funny how the manual doesn't list "speed" or "tire size" as an input.
@Strommen95

OK, I hear you.... I'll stand corrected... especially after input from Joe Z. you linked to.... 8-)

Guess my comments were pointing to... if you change tire size dramatically.... and the trannies shift points will be off til the transmission relearns over time, if possible for relearn.... if change is minimal... or you do a re-gear. Am I ok saying it like that?

I deleted earlier posts that may confuse readers....
 

DrJ

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Weird… my JL shifted all wonky when I went from 315/70/17 to 37/12.5/17. Didn’t shift normally until I corrected the tire size with JScan
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