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New 35s - speedometer change observation

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I had a set of 35’s installed this morning. I went from the 285/70-17 BFG KO2 that came on my rubicon to 315/70-17 KO2. The shop was an hour drive away, so I used waze on my way home to see how far off the speedometer would be. For the first 20-30 miles, it showed about a 4mph difference at 65 (speedo showed 65, waze showed 69).

There was a bit of traffic while working my way through Nashville, and when I got through downtown - both the speedometer and waze started showing the exact same mph. I looked at the tire pressure screen,, and all was still good there. It was very strange as they both read the same from 60 all the way up to about 80, which was as fast as I got today.

Anyone experienced the same? I have a programmer on order to adjust the tire size. I know the shop didn’t program the Jeep for the new tires.
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I had a set of 35’s installed this morning. I went from the 285/70-17 BFG KO2 that came on my rubicon to 315/70-17 KO2. The shop was an hour drive away, so I used waze on my way home to see how far off the speedometer would be. For the first 20-30 miles, it showed about a 4mph difference at 65 (speedo showed 65, waze showed 69).

There was a bit of traffic while working my way through Nashville, and when I got through downtown - both the speedometer and waze started showing the exact same mph. I looked at the tire pressure screen,, and all was still good there. It was very strange as they both read the same from 60 all the way up to about 80, which was as fast as I got today.

Anyone experienced the same? I have a programmer on order to adjust the tire size. I know the shop didn’t program the Jeep for the new tires.
Did you plug your phone in? Waze will use the Jeep's indicated speed instead of phone GPS if carplay is active.
 
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2manyhobbies

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Did you plug your phone in? Waze will use the Jeep's indicated speed instead of phone GPS if carplay is active.
yes, I did have it plugged in and was using CarPlay. What is odd is that it was clearly off for the first half of my drive. And then it wasn’t. I guess I’ll take it out later and not plug it in for CarPlay. Thanks for the info!
 

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I just asked my dealer to recalibrate for me when they put my wheels and 35s on (I had them wrapped into the financing). No problems with mine.
 

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I don't know what my true speed was with the OEM KO2s but after going to 315 KO2s I'm about 2mph faster than the spedo shows. Close enough for me. The 315 KOs are smaller than some 35s so that may make a bit of difference.
 

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I don't know what my true speed was with the OEM KO2s but after going to 315 KO2s I'm about 2mph faster than the spedo shows. Close enough for me. The 315 KOs are smaller than some 35s so that may make a bit of difference.
Just remember that everything is off, including your odometer.
 

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Your tires "grow" (increase in diameter) slightly with speed increase due to centrifugal force. This may account for the delta speed differential you are seeing for speedometer vs. GPS at various speeds. Also, the air temp/pressure inside the tire can affect the tire diameter somewhat. These variables affect the speedometer but do not affect the GPS speed.

However, you should have the speedometer calibrated for the larger tire size.
 
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Difference is a 285/70 and a 315/70, if both are true to size, is 5%. That just a couple MPH at 45. Probably pretty easy to miss depending on the calibration curve of the speedo and GPS error.
 
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Thanks for all of the comments. What I was most confused by was why it was off between Waze and the speedometer when I started the drive, but mid drive they matched. I think the explanation about Waze using the speedometer reading while using CarPlay makes the most sense.
My ECRI arrived today. It will be calibrated tomorrow.
 

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Thanks for all of the comments. What I was most confused by was why it was off between Waze and the speedometer when I started the drive, but mid drive they matched. I think the explanation about Waze using the speedometer reading while using CarPlay makes the most sense.
My ECRI arrived today. It will be calibrated tomorrow.
When you cross check the calibration, leave your phone disconnected. I want to say 34.5 or 34.6 worked best when I was on 35s.
 

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I ordered an ECRI to program my speedometer. Using a free gps app, I was able to estimate a scaling factor to apply to the current tire size in the PCM. I programmed the new tire size, and was pretty close after a drive up to 80mph. I calculated the final tire size, and compared that to the ECRI app’s suggestion, and they were within 0.03” of each other.

FWIW, my 315/70-17 tires on factory wheels at 40psi calculated to be 33.4” in actual diameter. That’s what makes the speedometer dead on accurate to the gps speed.
If my memory serves me well, the factory tire diameter in the PCM on my 4xe rubicon was 31.3” (285/70-17). The original and new tires are both BFG KO2, load range C.

A note on ECRI - I had a question about the app. Sent a note, and had a response within about an hour this morning.
 

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When you cross check the calibration, leave your phone disconnected. I want to say 34.5 or 34.6 worked best when I was on 35s.
I also went to 34.5” with the tazer after getting 315/70r17s.
I think the stock size in the computer for the 285/70r17s was 32.5”.
 
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The ECRI shows you what the Jeep is programmed at currently. So I know that the factory rubicon tires were programmed at 31.3” when I started. The app tells you the current setting and confirms the new one before it starts to update anything. It really is a neat app and product.

I just used math to calculate the new tire size. Your results may vary.
 

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I had a set of 35’s installed this morning. I went from the 285/70-17 BFG KO2 that came on my rubicon to 315/70-17 KO2. The shop was an hour drive away, so I used waze on my way home to see how far off the speedometer would be. For the first 20-30 miles, it showed about a 4mph difference at 65 (speedo showed 65, waze showed 69).

There was a bit of traffic while working my way through Nashville, and when I got through downtown - both the speedometer and waze started showing the exact same mph. I looked at the tire pressure screen,, and all was still good there. It was very strange as they both read the same from 60 all the way up to about 80, which was as fast as I got today.

Anyone experienced the same? I have a programmer on order to adjust the tire size. I know the shop didn’t program the Jeep for the new tires.
I experienced the same thing. I just replaced my stock Rubi on 285 KO2's with 315's and the first time on the freeway at around 65mph, the speedo was about 4mph off Waze. I thought Waze was using GPS for speed and trusted that was real mph. I accepted the difference and thought I'd need a tazer or other adjustment. But on the return trip home on the highway, Waze was within 1mph. I didn't know Waze recalibrate to in car readings and will check another way using GPS mph.
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