superchip tire size - JKowners.com : Jeep Wrangler JK Forum
 
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post #1 of 9 Old 06-09-2011, 05:32 PM Thread Starter
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superchip tire size

i just put on my new 315.70.17's and went to resize the tire size with my superchip and when i did it to 34.75( bc thats the clostest to a 315)i was like 5 mph off from my gps unit and the speedo, so i kept going down till i finally got to about 33.5 inch and bow im only about 1 mph off, should i worry about that or is that about the best i am goin to get......What are you guys running on your superchips with 35's?????

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post #2 of 9 Old 06-09-2011, 05:52 PM
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Very common for 35's to not measure 35", but closer to 33.5". I believe the flashpaq instructions even tell you to measure the tire instead of using the mfg's listed spec. Also, figure that 1mph difference between gps and dash to be about as good as you're going to get. You can keep playing with the tire size, but after quite a few tries I just gave up and settled for close enough. (the ProCal is more accurate as far as calibration goes)
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post #3 of 9 Old 06-09-2011, 11:03 PM
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Yup, brand new 35x12.5x15 BFG KM2 measured 33.5 inches tall. After my superchips, I'm right on. I drive from Las Vegas to San Diego sometimes 2 a month and the speedo is right on.

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post #4 of 9 Old 06-10-2011, 07:35 AM
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That's definitely normal if you're runnin' a KM2.

I'm runnin' a MT/R Kevlar and my 35x12.5-17's measure out to 34.25" on a 9.5"-10" wide wheel. If they were on an 8.5" wide wheel, I'd probably get closer to 34.5"-34.75".

Did you even attempt to measure them? Measure from the ground to the center of the wheel and double that. That's about as close as you're gonna get for your Superchips measurement.


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post #5 of 9 Old 06-10-2011, 08:14 AM
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lol my 37" km2s measure out to 35


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post #6 of 9 Old 06-10-2011, 08:28 AM
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Tape Measure and 5 seconds... not too hard dude....
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post #7 of 9 Old 06-10-2011, 09:28 AM
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What you want to do is to make a mark on your tire with the tire on your car and a corresponding mark on the floor. Move the car for one tire revolution and measure the distance. Divide that by pi (3.14) and that is the diameter of the tire under load. You can then "tweek" it in with your GPS to get your speedometer accurate again.

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post #8 of 9 Old 06-10-2011, 08:29 PM Thread Starter
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lol yeah sorry probley should have thought this through better but now its almost dead on thanks guys!!!!

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post #9 of 9 Old 06-11-2011, 10:35 PM
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Quote:
Originally Posted by OlMan View Post
What you want to do is to make a mark on your tire with the tire on your car and a corresponding mark on the floor. Move the car for one tire revolution and measure the distance. Divide that by pi (3.14) and that is the diameter of the tire under load. You can then "tweek" it in with your GPS to get your speedometer accurate again.
Why would you want to move the tire over to your car? Why not just do that same test while the tire is on the Jeep?


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