Jscan and Tire Size - Observation/Question

[sdk131]

So I am new to having a fully licensed Jscan version, but I have had other similar software products for over a decade when dealing with other vehicles. I just got all set up with the obd connector and cable and went in to check/adjust my tire size yesterday. I have a 2021 sport s with max tow (came stock with 245/75/17 tires). I had already installed my 285/70/17 JLUR takeoffs and was expecting to have to tweak the tires size to 31.75" or 32" with the jscan. However when I went in to the axle and tire module for the first time it was showing the reading from the ECU as 31.93". This made me wonder if the ECU had already calculated the tire diameter somehow via the sensors.......or if perhaps ths was the default setting? Just wondering if anyone else had noticed similar or could offer any additional insight.

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[sdk131]

@sdk131

JSCAN, cannot and will not see and configure anything. That setting you see is just the first setting you have to chose from on the list.

This was above the drop down list....and it said something like "current reading from the ECU". Here is a screenshot from the screen in Jscan on demo mode as I don't feel like hooking up all of the connectors again. Where it says "tire height 32.64" in the screenshot it said 31.93" when I was conencted to the jeep yesterday. The pulldown menu is below that.

 

[sdk131]

@sdk131

The wording in my post was poor....

First off your JT, cannot determine your tire size and automatically adjust for same.. That is why you program or have a dealer program for tire size.

JSCAN is not going to overview your JT somehow, and do setting changes automatically.

IF you want any change, you need to select and enable/disable the change.

Now, once you do program your tire size, speedometer will be correct.... ALSO, what will happen after you program for tire size, you JT, will do this automatically .... it will adjust shift points if you have an automatic transmission.

Ahh ok, thanks. The wording on the jscan screen led me to believe that it was reading the value from the ECU since it literally says "current value read from ecu". Perhaps it is also just a poor choice of words on the part of the jscan developer.

I also know that some cars do in fact have the ability to calculate an approximate tire diameter using abs and other sensors (my VW had this as a sort of crude tire pressure monitoring system). I did go ahead and manually set it to 31.75" as that was the actual measured height of my 285/70/17s as mounted.

 

[FutureOdin]

Ahh ok, thanks. The wording on the jscan screen led me to believe that it was reading the value from the ECU since it literally says "current value read from ecu". Perhaps it is also just a poor choice of words on the part of the jscan developer.

I also know that some cars do in fact have the ability to calculate an approximate tire diameter using abs and other sensors (my VW had this as a sort of crude tire pressure monitoring system). I did go ahead and manually set it to 31.75" as that was the actual measured height of my 285/70/17s as mounted.

JScan is/was reading the value that is currently set in the ECU. From the factory, it came set as 31.93"

 

[Tufelhundin]

I certainly believe my ECU was programmed for 285/70-17's from the factory.

With a stock sport S tires 245/75-17's I drove from La to Dallas (5.5 hrs) to picked up a set of Rubi tires and wheels from a forum member on the way I pulled out the iPhone app and noticed my mph's were off a bit., I had the rubi take-offs installed while I was in Dallas then drove back home. Using the same App I found my speedo was pretty much dead on with the 285/70-17 from the Rubi.

 

[sdk131]

JScan is/was reading the value that is currently set in the ECU. From the factory, it came set as 31.93"

That's good to know, thanks! LOL...guess there was no immediate need for me to go in and "correct it" then. That's a bit interesting, since the 245/75/17 factory tires were quite a bit smaller.

I actually already had a more advanced OBD scan tool that would have enabled me to read codes/troubleshoot, so I purched the JSCAN strictly for the tire correction and any other small setting tweaks I may need/want to do at some point. Oh well, now I have the jscan for other general uses, at least it was not a huge expense.

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