Problems programming tire size

[dstevens]

2024 X. Just gone from 315/70-17 to STT Pro 37s. 604 revs/mile to 564 revs/mile or 7% increase in circumference according to the specs.

I've tried and failed to program the new tire size. I've used JScan, HPTuners and the Tazer. In each case no matter what tire size I input the speedometer reads over by 20%. Going to a larger tire should make the speedometer under read.

I settled on using the Tazer and here was the procedure:
- undated tazer firmware
- key on, engine off
- get into the Tazer menu, go to Perf, then tire size
- change the tire size to around 36.7", accept new size. The dash shows some signs of programming with a few warnings that come and go.
- in the Tazer menu, select full reboot, wait the 2.5 minutes for the reboot
- key off
- get out of vehicle, close drivers door, take remote 100' away
- after 40 seconds the dash goes off, then wait an additional minute
- get in vehicle, start engine, stop engine
- get out and repeat the sleep procedure

Is there something obvious I am doing wrong? I've also run it on the freeway to see if that changes anything.

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[GATORB8]

Is the Tazer "remembering" your input tire size after the reboot/sleep cycles?

 

[Flip]

I would set everything back to default factory settings, unmarry the tazer and start over. Then I would try the Jscan first because when you disconnect the Jscan it doesn't lose the settings.

 

[Grayhound]

I like to use the tazer for the gateway access and use jscan to make the changes that the tazer allows onto the jeeps computer.

They're is a setting in the tazer menu that bless you write to the computer via the obd2 port. If that is enabled the jscan can do all the changes you want

 

[dstevens]

The tazer was remembering the tire size.

I unmarried the tazer. Then did a reset and a couple of sleep cycles. Programmed the tire size in jscan to 36.5" (spec is 36.77" I think). Did a reset and then two sleep cycles. Drove for about 20 minutes, including freeway at 60-70 mph. Speedometer still out by 9 mph @ 60 mph.

Several hours later I got in the Jeep and drove on the freeway and the speedometer was 100% accurate. So accurate that I think that it was calibrating using GPS. With the cruise control on 70 mph the GPS speed shows 69.9 - 70.2 mph.

So my conclusion is that the either Jeep does a GPS calibration with some sort of constant speed or distance requirement or I just nailed the tire size and it needed more sleep cycles.

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