JScan Tire Size Oddity - Help?

[rcadden]

So I have a Sahara Altitude. Came from the factory with 32" Duratracs. Swapped with 33" KO2 Rubicon takeoffs back in December. Have had 2 oil changes since then, at the dealership (cause free).

Finally had some change to get an adapter and whatnot and got all kitted up with JScan, so I could tell the Jeep it's turning 33s instead of 32s.

Get it all hooked up, etc, no problem. Measure my tires as JScan recommends, ground to top of tire. 31.5" on the nose (seems low, but I do run at 35psi)

I load up JScan and it says my tire size is already set to 32.25".

So....not real sure here.

1) Would the factory have set it to 32.25" instead of actually measuring the tire?
2) If not, then is it possible that one of the two dealerships hooked into the OBDII while doing my oil change, noticed the tires didn't match the system, and updated it "for me" (without mentioning it)?
3) If my system reads 32.25", but my tires measure 31.5", which should I program to have things setup most accurately?

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[mferrara91]

So I have a Sahara Altitude. Came from the factory with 32" Duratracs. Swapped with 33" KO2 Rubicon takeoffs back in December. Have had 2 oil changes since then, at the dealership (cause free).

Finally had some change to get an adapter and whatnot and got all kitted up with JScan, so I could tell the Jeep it's turning 33s instead of 32s.

Get it all hooked up, etc, no problem. Measure my tires as JScan recommends, ground to top of tire. 31.5" on the nose (seems low, but I do run at 35psi)

I load up JScan and it says my tire size is already set to 32.25".

So....not real sure here.

1) Would the factory have set it to 32.25" instead of actually measuring the tire?
2) If not, then is it possible that one of the two dealerships hooked into the OBDII while doing my oil change, noticed the tires didn't match the system, and updated it "for me" (without mentioning it)?
3) If my system reads 32.25", but my tires measure 31.5", which should I program to have things setup most accurately?

If you have 285/70/17 tires, the tire diameter is 32.7. I believe JScan has 32.75 as an option. That will give you the correct size and speedometer will be spot on.

 

[The Last Cowboy]

It seems that all Wranglers that are eauipped with 245/75/17s, 255/75/17s or 285/70/17s are all programmed the same way from the factory. I know my factory 255s read about 3 MPH fast at 70. So if I go to 286/70/17s the speedometer should be accurate.

 

[rcadden]

Sorry, still a bit confused.

So my current reading is 32.25. JScan says to measure floor to top of tire, which is 31.5 for me on 33s. But you're saying I should run it up to 32.75 for 33s?

 

[Gregj]

I wouldn’t get too hung up on the numbers, the real question is your Speedo reading correctly? Use GPS to see if it is correct, if not make adjustments from there.

Gregj

 

[Steph1]

Seems like the factory are now using the tire manufacturer announced diameter which is 32.25 for a 255 stock Sahara tires. My 2018 Sahara was set at 31.25 in the computer with the stock 255 tires.

What you want is a read of the correct speed and optimum transmission shifting. So go with the Jscan way of measuring and get on the highway to see the difference between your Jeep's speedometer and a GPS phone app. Don't forget to disable Bluetooth otherwise your phone will automatically match the jeep's speed, and adjust tire size until the speeds match.

 

[falcon241073]

I wouldn’t get too hung up on the numbers, the real question is your Speedo reading correctly? Use GPS to see if it is correct, if not make adjustments from there.

Gregj

I use a speedometer app. I found out by accident that if I have my iPhone plugged in the usb for Apple carplay then the speedometer app shows what the Jeep says. If I unplug the app it shows what the app/phone is reading from its internal gps sensor/gps satellites.

 

[rcadden]

So I downloaded a GPS app and it shows that I'm actually moving a little faster than my Jeep's GPS reads. When the Jeep says 70, I'm going 72. The different is slightly more pronounced at lower speeds.

 

[mferrara91]

Sorry, still a bit confused.

So my current reading is 32.25. JScan says to measure floor to top of tire, which is 31.5 for me on 33s. But you're saying I should run it up to 32.75 for 33s?

That is correct. I have already done this before with my Jeep using JScan. I swapped out for a set of Rubi takeoffs. 285/70/17 measure 32.7 diameter. Set JScan to 32.75, checked speed against GPS and it was spot on. Prior to it was off by about 1-2 MPH. Just recently did it again when I switched to 35s.

https://tiresize.com/calculator/

 

[mferrara91]

So I downloaded a GPS app and it shows that I'm actually moving a little faster than my Jeep's GPS reads. When the Jeep says 70, I'm going 72. The different is slightly more pronounced at lower speeds.

Trust me, change the tire size to 32.75.

 

[rcadden]

Thanks all - will jump up to 32.75 later today.

 

[GrayWolf.Overland]

I had the same gotcha when i changed tires last year and updated the settings. Jscan read the default as 32.25 for my Goodyear Wrangler Adventure ATs 255/70/18

I followed Jscan instructions and set the tire size of my Grabber ATX 285/70/17 to 31.5 (measured height at 32 psi). The speedo was off by 4 or 5 kms.

Just like others said i then set Jscan at 32.75 which is the closest to the diameter of my tire (32.8). Tested again with gps speedo and it's spot on.

I think there is an offset adjustment from factory somewhere in the firmware.. It seems to want us enter the actual tire size.

 

[rcadden]

Just closing the loop - updating to 32.75 worked.

GPS app matches my speedo now, and my MPG is back to reporting the 20-23 avg I was getting before I put the 33s on.

Side note: was thinking through this logically on my drive home this morning - the JScan suggestion of measuring the diameter of the tire doesn't even make sense. They recommend measuring from the floor to the top of the tire, and putting in that.

However, this measurement will be significantly impacted by your chosen PSI, etc. It's effectively measuring the thin part of an oval, at that point - not the diameter of a circle. The circumference doesn't change, regardless of the shape of the tire. So on a 33" tire, regardless of whether it's at 30 PSI (and more of an oval) or 41 PSI (and more of a circle), your Jeep is still turning the full circumference of a 33" tire.

 

[jaymz]

Just closing the loop - updating to 32.75 worked.

GPS app matches my speedo now, and my MPG is back to reporting the 20-23 avg I was getting before I put the 33s on.

Side note: was thinking through this logically on my drive home this morning - the JScan suggestion of measuring the diameter of the tire doesn't even make sense. They recommend measuring from the floor to the top of the tire, and putting in that.

However, this measurement will be significantly impacted by your chosen PSI, etc. It's effectively measuring the thin part of an oval, at that point - not the diameter of a circle. The circumference doesn't change, regardless of the shape of the tire. So on a 33" tire, regardless of whether it's at 30 PSI (and more of an oval) or 41 PSI (and more of a circle), your Jeep is still turning the full circumference of a 33" tire.

Yep. My Speedo was off about 2mph when I measured as recommended by JScan. I measured my spare, punched in that number and the speedo is now spot-on.

 

[Rhinebeck01]

@rcadden

Do check at different mph speeds .... say 35, 55, 75 mph using a GPS app that is on a smart phone not tethered to the JL. Just be sure across the board your speedo is reporting correctly at all speeds.

As time go by... as your tires get worn / real worn you may possibly have to adjust for tire size again...

JSCAN is very good but you will see features that do not work and a tad bit of misinformartion lets say... Always best to come here and ask fellow user's before you take the leap in regard to settings done with JSCAN.

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