What size tire should I use for J-Scan?

Chibluefreedom

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just bought new tires. Noticed shifting points have changed, but not much. Stock tires were 32.1 diameter(goodyear wranglers). New tires are same diameter just beefier (Bfg ko2). I should be getting
The Vgate obd tool in the mail anyday now and ill be installing the jscan app. Im wondering What tire size setting i should use? Both tire diameters are the same but dont perform like it.
 
Is your spare tire matching? If so, measure the spare and see what size it measures out at, as it's often different than what the manufacturer says (especially with BFG).

Use the measured size for the J-Scan app, as that will be the most accurate (y)

For instance, my 315 MFG KO2 spare measured exactly 33.3" while mounted on the spare tire carrier. Always measure the tire without weight on it of course (hence the suggestion for measuring the spare).
 
I changed the tire size to what it should be and then went for a drive using my Garmin to check my speed with the Jeeps Speedometer.
 
Are the sizes you quote measured or manufacturers specs? I've found it makes sense to measure from the ground to the center of a tire installed and under load on my JK, then double it, and use that number. It will be slightly less than measuring from the ground to the top of the tire but I've found it more accurate as a rolling tire is always under load for its circumference.
 
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i thought you were suppposed to measure from the top of a loaded tire to the ground for exact tire size - not the unloaded spare.
 
i thought you were suppposed to measure from the top of a loaded tire to the ground for exact tire size - not the unloaded spare.

A tire on the ground will have a flat spot on it where it sits on the ground, and depending on the tire pressure will change the amount of flat area.
 
the instructions in the JScan app say to measure from the ground to the top of a loaded tire.
 
Ya, I would go by measuring to the ground and use a level to be as accurate as possible. Of course a level ground (as best as you can locate) is key as well.

Real world tire measurement, since it’s on the ground. End of day, you can check it with a speed app, or WAZE, to get it as accurate as possible. I say within 1 mile you’d be fine. But to be clear, that’s my process. Whether it’s 100% Im sure can be questioned. I’ve noticed that when getting up in the higher speed range the speedo may be off by 1 mile, at around 70 MPH.
 
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My speedo was about 3 mph fast at 55. I went from stock to 285/70s and now 55 is 55. 30 is 31 and faster than 55 I don’t care much. Checked with a gps app on my iPhone. My shift points did change a little. Over time the transmission relearned and it seems normal again.
 
Ya, I would go by measuring to the ground and use a level to be as accurate as possible. Of course a level ground (as best as you can locate) is key as well.

Real world tire measurement, since it’s on the ground. End of day, you can check it with a speed app, or WAZE, to get it as accurate as possible. I say within 1 mile you’d be fine. But to be clear, that’s my process. Whether it’s 100% Im sure can be questioned. I’ve noticed that when getting up in the higher speed range the speedo may be off by 1 mile, at around 70 MPH.
I see differences in waze from other apps.
 
Do what @Chris said and then go drive it when a speedometer app. I had to hone mine in after I measured the spare. it was slightly off.
It's solid instruction. With 35s at factory settings the speedometer was 5mph fast at 70mph. Reprogramming the computer with a Supercal to 35" tires made it 3mph slow at 70mph. Measuring the spare to the mid point and multiplying by 2 and updating the computer settings has the speedometer on the money. 👍👍
 
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It's solid instruction. With 35s at factory settings the speedometer was 5mph fast at 70mph. Reprogramming the computer with a Supercal to 35" tires made it 3mph slow at 70mph. Measuring the spare to the mid point and multiplying by 2 and updating the computer settings has the speedometer on the money. 👍👍
It's solid instruction. With 35s at factory settings the speedometer was 5mph fast at 70mph. Reprogramming the computer with a Supercal to 35" tires made it 3mph slow at 70mph. Measuring the spare to the mid point and multiplying by 2 and updating the computer settings has the speedometer on the money. 👍👍
I’ll give it a try. Odd, though, because I went by the AEV manual that came with my device.
 
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It's solid instruction. With 35s at factory settings the speedometer was 5mph fast at 70mph. Reprogramming the computer with a Supercal to 35" tires made it 3mph slow at 70mph. Measuring the spare to the mid point and multiplying by 2 and updating the computer settings has the speedometer on the money. 👍👍
This, along with my stock tire experience (58 was 55), tells me the speedometers are slow from the factory on purpose.
 
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