2021 JLUR speedometer calibration?

[Headbarcode]

I respectfully disagree that tire circumference is a primary concern, but appreciate the thoughtfulness of your explanation.

Can any of these after market devices adjust the RPM shift targets on the power band? Can they alter the pressure thresholds in the Torque Convertor? Can you use them to tailor the algorithm for the specific curb weight of your modified vehicle, and the implied inertia of the vehicle?

The auto transmission has plenty of information to work well without prioritizing tire size. Changing tire size doesn't change the power band of the engine. The torque convertor doesn't need to know what tire size you have to do its job well. There doesn't seem to be any opportunity to tell the Jeep what the gross vehicle weight is, so the inertia during acceleration, deceleration, and cruising, on flat or angled grades, seems to be more or less generalized rather than specific.

If you were shifting a manual with newly introduced changes such as bigger tires or different gears, you would still shift to make best use of the engine's power band , just like the automatic transmission does. You would know exactly where to shift because... you know... you know when to shift without thinking about tire size, or gear ratio, or any of that.

If you wanted to you could discern that something about your Jeep had changed, e.g. the land speed where the changes occur are now at different rates, but that would not have to equate to a problem unless you made it one in your imagination.

It's nice to have an accurate speedometer, but IMO a modified Jeep can be enjoyed with out hacking the software.

All the best!

I have to agree to disagree, as I wasn't passing on thoughts and feelings.

Curb weight varies, and translates as engine load. The focus in this thread are the set points. Also, we're not talking about engine and transmission tuning, but simply a body control module programmer. This is a well known reality. No place for imagination in a technical conversation.

A simple analogy would be a programmable coffee machine. As sophisticated as it could be, it still requires an accurate clock because it doesn't know that the sun's coming up on its own.

Algorithms require both variable and set data points. Each one has equal importance to the collective, when looking for the best results.

Sponsored

 

[Yellow Cake Kid]

With my Jeep running the larger tires, without a new calibration, I would be more likely to acknowledge that I have undermined the effectiveness of the ABS and Traction Control, which in my case are two features I don't make much use of, but could be of great interest to others.

 

[Yellow Cake Kid]

...No place for imagination in a technical conversation....

...Algorithms require both variable and set data points. Each one has equal importance to the collective...

Post 230

 

[Headbarcode]

Post 230

Yeah, I'd like an explanation on why speed has zero affect on transmission shift points. Furthermore, if it has nothing to do with it why does changing the tire size have a dramatic and instant effect on shift performance?

 

[Headbarcode]

With my Jeep running the larger tires, without a new calibration, I would be more likely to acknowledge that I have undermined the effectiveness of the ABS and Traction Control, which in my case are two features I don't make much use of, but could be of great interest to others.

Strommen posted a link to an article in another thread. Here's a screenshot that clearly explains how incorrect calibration will throw off the nanny systems, thus negatively affecting shift performance. Funny thing is he posted that article to reinforce that tire size calibration has zero affect on shift points.

 

[Strommen95]

Strommen posted a link to an article in another thread. Here's a screenshot that clearly explains how incorrect calibration will throw off the nanny systems, thus negatively affecting shift performance. Funny thing is he posted that article to reinforce that tire size calibration has zero affect on shift points.

screenshot_20211104-202640_chrome-jpg.530714 (720×1600) (jlwranglerforums.com)

The calibration you're referencing is referring to the vehicle speed sensor, aka the output shaft speed sensor. It's true that a bad VSS will affect shift points but the VSS's input is from a sensor within the transmission. What's read digitally is not tied to this sensor. Speed is a factor with the transmissions shift points. It's read by the VSS, not what's on the display though and this is where your thinking is flawed.

There's nothing in the article that supports what you're saying. It's amusing that in the same paragraph discussing diagnosing shifting issues, nothing is said about calibrating the speedometer/tire size. The only thing that's funny here is sharing excerpts in "support" of your argument that you don't quite understand.

 

[WRANBO]

I know that there is a lot of coverage on this already but I just wanted to add my experience as well for the next person.

I have a 2019 Sahara Unlimited, 2.0, 8 speed, 3:45 gearing.

Installed 285/70/17 Motomaster AT3s on JK Rubi rims (from my old 2017 Sport). Tires fit perfectly fine on a 7.5” factory rim. Tires clear fenders, no rubbing, or odd noises.

Transmission still shifts smoothly into all gears, including 8th (I know some seem to be having the issue of not seeing 8th at highway speeds when going up a tire size). I did not recalibrate or reset my TCM.

The vehicle feels a bit heavier while driving but certainly not in a reduced performance way (when turbo kicks on, she still flies).

I also bought the same tire for the spare and it fit fine with no adjustments being made to the 3rd brake light bracket (as some have suggested).

Definitely gives a tuff look to the Jeep. I may want to eventually get some Rubi springs and shocks to add some height in the fender wells. In my opinion, the 33” tires don’t look crammed as some have said, but I think balanced is the better word. Just so you know, your spring rate will also be higher (which allows more space between tire and fender) if you have the tow package (which I don’t).

I am pleased with my decision to go the 285/70/17 route. Will add pictures tomorrow.

Sponsored