Am I overthinking this?

aldo98229

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You got two separate things going on:

  1. You need to measure the distance from the center of your wheel to the floor and multiply it by 2. That will give you the closest read to your tire’s true size
  2. I noticed when I calibrated my speedo, that the built in tire sizes that come from the factory are off. My tires measure 34 inches mounted; I believe I set the tire size at 33.5 to get an accurate speedo reading
Good luck!
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Yogi

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Wabujitsu

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I purchased the ECRI device. It has a function, based on GPS and other sensors, that determines the actual, true tire size for you, within a couple mile’s drive at a fairly consistent speed. I used this instead of the manual input of the true diameter, and it has turned out to be remarkably accurate; it is spot-on!!!

Not only is this device less expensive than the Tazer models, it also does many other calibrations for your vehicle. https://ECRI.app
 

Wabujitsu

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I purchased the ECRI device. It has a function, based on GPS and other sensors, that determines the actual, true tire size for you, within a couple mile’s drive at a fairly consistent speed. I used this instead of the manual input of the true diameter, and it has turned out to be remarkably accurate; it is spot-on!!!

Not only is this device less expensive than the Tazer models, it also does many other calibrations for your vehicle. https://ECRI.app
Also, after making your calibrations, you can unmarry and remove the device from your vehicle. The settings are permanently stored in your on-board OEM computers. The dealership will NOT be able to tell that you made the adjustments with an aftermarket device!

Additionally, you can use one device for multiple vehicles and it will store each vehicle profile. It only requires a one-time, permanent unlock code for each vehicle at $49.99.
 

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I found that if you look up the specs and use the revolutions per mile, you'll be pretty close.

63360 ÷ revs per mile ÷ 3.14 = rolling diameter
 

DavidArmen

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#1 and #3, but not in any specific order.

No GPS speedometer or doppler sign is perfect, but entering a value that is larger than the actual, physical tire size just to get it to match readings eventually adds up on your odometer because you're adding mileage that doesn't exist.

I don't know what size Jeep calibrates non-BFG tires, but they don't set it to the rated size on the Rubicon. The 33" KO2s, for example, typically measure 31.5" on load and they set it to this value.
One can easily check what size the Jeep calibrates their tires with the tazer! As long as you haven’t changed the tire size, the default number will be the factory calibrated size. I happen to have a 2021 JLUR with the factory 33” BFGs and a tazer and I am now checking my tazer as I type this..

My tazer shows my stock tires as 32.67”
image.jpg
 

aldo98229

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One can easily check what size the Jeep calibrates their tires with the tazer! As long as you haven’t changed the tire size, the default number will be the factory calibrated size. I happen to have a 2021 JLUR with the factory 33” BFGs and a tazer and I am now checking my tazer as I type this..

My tazer shows my stock tires as 32.67”
image.jpg
Yes, I too noticed a discrepancy between actual tire size and what my Tazer shows I need to enter in order to get an accurate speedometer reading.

For the speedometer to be spot-on, I had to enter a tad smaller size on the Tazer screen than I measured the actual tire to be.
 

DavidArmen

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Yes, I too noticed a discrepancy between actual tire size and what my Tazer shows I need to enter in order to get an accurate speedometer reading.

For the speedometer to be spot-on, I had to enter a tad smaller size on the Tazer screen than I measured the actual tire to be.
Not surprising at all because tires are always smaller than what the sidewall size claims them to be. So the calibration will always be a smaller diameter than the claimed size.

The issue here is that I have not read anywhere on these forums what the most correct way is of physically measuring the actual tire size as far as the jeep’s computer is concerned.
 

aldo98229

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Not surprising at all because tires are always smaller than what the sidewall size claims them to be. So the calibration will always be a smaller diameter than the claimed size.

The issue here is that I have not read anywhere on these forums what the most correct way is of physically measuring the actual tire size as far as the jeep’s computer is concerned.
I measured from the floor to the middle of the hub and doubled it.

The computer still needed a tad smaller number for the speedo to be correct.

BTW, my JLU came from the factory with the speedometer off by 1.5 MPH; it became spot-on when I swapped the OE 255/70R18 tires (32.1 inches) with 275/70R18 KO2s (33.2 inches).
 

CT_LFC

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#3, but instead of snooze fest MLB i suggest some NHL playoff action.
 
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