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Old 03-13-2014, 11:47 AM   #1
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Inaccuracy of factory program settings

This morning while exploring the TUPS programming with the Superchips Flashpaq; I discovered the factory or dealership had programmed the tire size at 30.75". My Wrangler Sport S originally was shipped with 255x75R17 (32.1") tires; so I it is unclear why the program setting wasn't around 32". I am sure this setting had an impact on the automatic transmission shift points and the speedometer readout.
Figuring the tires are worn.... I changed the tire size to 31.75". I will drive the Jeep for a few days to see what impact this change has on vehicle performance.

Anyone else find a problem with inaccuracies in factory programming?

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Old 03-13-2014, 11:54 AM   #2
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Use a good GPS to check your speedo. Then you'll know if they screwed up or not.

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Old 03-13-2014, 12:01 PM   #3
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You have to go by the actual size the tires measure while mounted on the vehicle on the ground. It will always be less than the size of an unmounted (off the rim) tire.
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Old 03-13-2014, 12:02 PM   #4
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32" tires are not actually 32" under weight. I don't know about the Flashpaq instructions, but the AEV ProCal says to take a tape measure and get the actual height. That being said, I've had the best shifting points from getting closer to 32" setting, I backed it down to 31.5"
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Old 03-13-2014, 12:14 PM   #5
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It may not have a 16" radius under weight, but the roll out distance would have to be the same regardless of how much you smoosh the tire down., which for the speedometer is what's important. Correct?
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Old 03-13-2014, 12:31 PM   #6
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What matters is the RPM not the length of the perimeter.
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Old 03-13-2014, 01:25 PM   #7
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My Speedo was off from factory. Went from 32"s to 33"s and then checked Speedo against my Garmin gps. Speedo was dead on after new tires.
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Old 03-13-2014, 01:33 PM   #8
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If your using a gps to set your speedo, get a close match at 75 mph +/- . Maybe everyone already knew this?
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Old 03-13-2014, 09:06 PM   #9
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This evening I contacted a friend of mine with a stock 2013 2dr JK. Since his 255x75R17 tires are almost new; I asked him to come over so I could measure them. Using a level and measuring tape I verified how tall his tires are with 35 PSI in them; his tires measure 31.25".
Tomorrow I will verify the MPH with a GPS and then I will correct the tire size in the TUPS programming.

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