? About speedometer and 35"tires - JKowners.com : Jeep Wrangler JK Forum
 
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post #1 of 8 Old 08-13-2015, 03:54 PM Thread Starter
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? About speedometer and 35"tires

Ok, I searched for my issue and found everything but my answer to my issue, lol. First off I will say I'm basing my correct speed off my nav which is connected to the vehicle speed sensor/signal. I have 35"(315/70/17) Duratrac tires on 17" rims, and I bought a Superchips Flashcal.

Prior to the Superchips my speedo would say 55mph and I was really going 59-60mph. Fast forward to my buying the Superchips and changing tire size to 35". Now my issue is exact opposite, if I'm going 55mph on speedo, I'm actually going 50-51mph and it gets worse the faster I go, if I'm up to about 75mph on speedo, I'm actually only at 67-68mph.

What gives??

Should mention it's a 15 Sport

Last edited by Shawn1112; 08-13-2015 at 03:57 PM.
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post #2 of 8 Old 08-13-2015, 04:56 PM
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Surprise, you're 35" tire isn't actually 35"s tall.

My 37's are only 36.3 by my GPS speed.

2014 JKURx
35" Nitto Ridge Grapplers
2 M - KG4ZFC
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post #3 of 8 Old 08-13-2015, 09:35 PM
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Yup. Measure your tire. That Duratrac is just under 34".


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post #4 of 8 Old 08-14-2015, 06:32 AM
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I have the same tires and I set the size at 33.75 and the speedo is dead on

2007 rubicon unlimited
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post #5 of 8 Old 08-14-2015, 08:50 AM
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My same size Duratracs on a '14 2dr Sport measure 33.5" at 28psi.
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post #6 of 8 Old 08-14-2015, 09:19 AM
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Set your tires to your preferred pressure. Then measure from the center-cap to the ground. Double that measurement. That is the effective diameter that your speed sensors are picking up.
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post #7 of 8 Old 08-16-2015, 05:19 PM Thread Starter
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Appreciate the help, will go out in the garage and measure right now
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post #8 of 8 Old 08-18-2015, 04:33 PM Thread Starter
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Ok, I tried 33.75 and it was pretty damn close, only off by 1 mph. Did the measurement from center cap down, came out to 17", changed tire size to 34" and it's dead nuts on now. Thanks for all the help
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