Self Correcting Speedometer

sfadchi

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When I first got my Jeep I immediately ordered 315/70/17 KO2s. I know they are that much bigger but the speedometer was still dead on accurate. I had the dealer reprogram for the new tires anyway just to be safe.

Now I'm selling those rims and tires and put on new 37/12.5/17 SST Pros. On Saturday when I put them on the speedometer was off quite a bit. Today jumping in and I felt like I was keeping up with the speed of traffic and felt it was more accurate so I pulled out my iPhone speedometer app. Sure enough, it's dead on accurate again. Anyone else notice this?

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Martindfletcher

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When I first got my Jeep I immediately ordered 315/70/17 KO2s. I know they are that much bigger but the speedometer was still dead on accurate. I had the dealer reprogram for the new tires anyway just to be safe.

Now I'm selling those rims and tires and put on new 37/12.5/17 SST Pros. On Saturday when I put them on the speedometer was off quite a bit. Today jumping in and I felt like I was keeping up with the speed of traffic and felt it was more accurate so I pulled out my iPhone speedometer app. Sure enough, it's dead on accurate again. Anyone else notice this?

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Mine never corrected after even a thousand miles had to use the tazer on mine.
 
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sfadchi

sfadchi

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Mine never corrected after even a thousand miles had to use the tazer on mine.
Do you have the 8.4" screen with Nav? Wondering if that has anything to do with it.
 

Penance81

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Oddly enough mine did this. I took it in to the dealership a few times to have them correct it but they were having trouble figuring out how to go through the process. They swear they never hooked it up and made any changes but one day I checked my vehicle and it was correct based on my iPhone and a road side speed sign. I had checked it before and it was off about 5mph at 60. I do have the 8.4" screen in mine.
 

LennyV

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There was a thread about this a while ago. It seems the speedo initially starts off at the old tire size then corrects itself after some driving. Once you turn off the Jeep it loses the new tire size. Once you start driving again it self-reprograms again.
 

Sloxkhart98

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Mine auto adjusted when I put 37’s on. I have the 8.4 w/ nav. But the gears never shifted right until I set the correct tire size with the FlashCal.
 

Martindfletcher

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Do you have the 8.4" screen with Nav? Wondering if that has anything to do with it.
Yes I have 8.4 with NAV. As noted below I also saw a big difference in shift points after correcting tire size. Previously it would stay in lower gears too long on freeway. Wasting even more gas.
 

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Just put 315/70/17 on mine and got out speedometer ap to see how much it’s off. (I have Manual so I wasn’t worried about shift points and it’s not really that hard to get used to when you are used to the difference at certain speeds)
Anyhow thought I was losing it because it was pretty much dead on with app. Made me want to look at tires to make sure they were the right size. I also have 8.4 with nav
 

offcamber

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It's your imagination. The speedo does not automatically calibrate to tire size. That is complete nonsense. Sorry, but it doesn't.
 

Spank

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There was a thread about this a while ago. It seems the speedo initially starts off at the old tire size then corrects itself after some driving. Once you turn off the Jeep it loses the new tire size. Once you start driving again it self-reprograms again.
What? No. This is not a thing. Your Jeep doesn't magically know you have different tires on it.

Your speedometer is more accurate at lower speeds, yes, but when you go faster, typically above 40mph, it becomes less so. Your speedometer isn't off by a set value, but a percentage.

The only way to get an accurate speedometer after changing tires is to adjust the tire size value through the dealership or a programmer. And don't blindly just put the tire size on the sidewall in. You'll want to actually measure the tire when it's mounted. Many tires, like BFG for example, can end up being an inch or more shorter than their rated size.
 

JeepPunk

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What? No. This is not a thing. Your Jeep doesn't magically know you have different tires on it.

Your speedometer is more accurate at lower speeds, yes, but when you go faster, typically above 40mph, it becomes less so. Your speedometer isn't off by a set value, but a percentage.

The only way to get an accurate speedometer after changing tires is to adjust the tire size value through the dealership or a programmer. And don't blindly just put the tire size on the sidewall in. You'll want to actually measure the tire when it's mounted. Many tires, like BFG for example, can end up being an inch or more shorter than their rated size.
Well my Jeep is pretty magical lol. I’m super skeptical of this myself; however I’ve used this exact same method to calculate speedo error in a pickup after going up in size and my predictions based on manufacture actual diameters from tire rack where very close to what my speedometer app showed.

Factory Rubi (33”) are listed as 32.8” and my new ones are 34.6”. And while there may be some difference based on my wheel diameter and what air pressure I’m at it should be close.
So my actual speed should be approx. 5.5% higher than what Jeep shows. At 65 I should actually be going about 68.
BUT at 65 Jeep speedo my ap actually read 64? So maybe there was high solar flare activity yesterday, the gps satellite was hacked by Russians, or maybe it was just a dream.
It was weird though and thats why I searched the forum yesterday.
Defineatly will be looking into it further because I’d rather not get a ticket “thinking” it was right.
 

sjl136

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One day I too thought my speedo had self corrected for my 315/70 17 K02s. Then I realized my iPhone was plugged into the usb port, and waze and my GPS based speedo app were all matching the JL speedo almost exactly. I then unplugged my iPhone, and both waze and the speedo app went back to being off from the speedo. I'm curious if those reading the same speed as their JL speedo are plugged into the usb? I will have to experiment with this again to be sure, but could the JL be feeding the speedo data through the usb/ carplay interface, making it look like the speedo self corrected? My apologies if this has already been covered elsewhere.
 

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What? No. This is not a thing. Your Jeep doesn't magically know you have different tires on it.

Your speedometer is more accurate at lower speeds, yes, but when you go faster, typically above 40mph, it becomes less so. Your speedometer isn't off by a set value, but a percentage.

The only way to get an accurate speedometer after changing tires is to adjust the tire size value through the dealership or a programmer. And don't blindly just put the tire size on the sidewall in. You'll want to actually measure the tire when it's mounted. Many tires, like BFG for example, can end up being an inch or more shorter than their rated size.
It is FM... @Spank ster...just admit it, bro...FM!!! (F***in' Magic) :cwl: <just giving you a hard time> :bandit:
 

JeepPunk

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One day I too thought my speedo had self corrected for my 315/70 17 K02s. Then I realized my iPhone was plugged into the usb port, and waze and my GPS based speedo app were all matching the JL speedo almost exactly. I then unplugged my iPhone, and both waze and the speedo app went back to being off from the speedo. I'm curious if those reading the same speed as their JL speedo are plugged into the usb? I will have to experiment with this again to be sure, but could the JL be feeding the speedo data through the usb/ carplay interface, making it look like the speedo self corrected? My apologies if this has already been covered elsewhere.
I’m almost always using CarPlay so?? Lol who knows. I’ll try without tomorrow.
 
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