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Programming tire size

biplaneguy

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This weekend I swapped my base Sport's wheels and tires with Mojave takeoffs, which are about 4% larger diameter. Got a BT dongle and security gateway bypass on the way to program the computer with Jscan for the new size.

Question: The tire size programmed obviously affects the speedometer and odometer, but does the computer use it any other way (like determining the shift points or fuel or timing mapping)? IOW will programming the tire size (or not) have any effect on performance?
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Jefe1018

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If it’s a few days you’ll be fine… the shift points will be adjusted once you adjust the Speedo.
 

Vtur

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I didn't wanted to racked up mine odometer, so i set mine new tires to match with gps and than set it to 1/4-3/8" smaller as tires can only wears down over times. GPS was spot on after about 20k miles on the tires.
 

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There should also be a selection in JScan to reset the transmission adaptive learning. I would suggest you do that after you get the tire size dialed in.
 

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biplaneguy

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Does the computer automatically adjust the tire size based on GPS data? Reason I ask is, I checked it with Jscan (read only, I haven't hooked up the security gateway bypass yet) and it's showing 32.25; I thought the OEM 245/75R17 tires were 31.5 (the new ones are 32.8 according to the specs; I haven't actually measured them).

Though oddly, the Tirerack website says the new tires are 634 revs/mile which works out to 31.8 diameter... the old ones were 661 which works out to 31.5 which agrees with the stated diameter.

There should also be a selection in JScan to reset the transmission adaptive learning. I would suggest you do that after you get the tire size dialed in.
I didn't see that; where is it?
 

Stan H

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Does the computer automatically adjust the tire size based on GPS data? Reason I ask is, I checked it with Jscan (read only, I haven't hooked up the security gateway bypass yet) and it's showing 32.25; I thought the OEM 245/75R17 tires were 31.5 (the new ones are 32.8 according to the specs; I haven't actually measured them).

Though oddly, the Tirerack website says the new tires are 634 revs/mile which works out to 31.8 diameter... the old ones were 661 which works out to 31.5 which agrees with the stated diameter.



I didn't see that; where is it?
There is a video on you-tube and it is shown exactly which section o Jscan it's in .. may be under transmission/tools
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