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Discussion Starter #1
If you saw my previous post, I upgraded the tires and wheels on my JGC using a set of the 2019 Wrangler Sahara tires and wheels. After putting on the larger tires, my speedometer was no longer correct. I was able to correct it using AlphOBD. Here are the detailed instructions for anyone else that would like to do the same.

My setup
2014 Jeep Grand Cherokee Limited V6
Changed tires to 255/70-18 Bridgestone Dueller A/T
Purchased OBDLink MX+ and AlphaOBD (other adapters may work, check the AlphaOBD web site for a list of adapters)

How to best check if your speedometer is correct:

Compare your Jeep speedometer to a Speedometer app on your SmartPhone that uses the GPS. I mounted my phone to the dash using a vent mount so that the phone speedometer and the dash speedometer are side by side. The app I used was called GPS Speedometer and Odometer by COOL NIKS in the Google Play store. I liked the app, so I purchased the Pro version for $1.10.

Take your Jeep for a drive and find a long straight stretch of freeway where you can set the cruise control. The GPS speed is updated once per second, so the changes in speed lag behind the actual speedometer if you are speeding up or slowing down. That's why maintaining a constant speed is necessary to compare the Jeep speedometer to the GPS speedometer.

For me, when my Jeep speedometer read 65 MPH, I was actually going 68 MPH after putting on the larger tires.

Here is how to make the correction:

Setup OBDLink MX+ with OBDLink app to make sure the OBDLink adapter is working properly first

Connect to Jeep with ignition in Run, but engine not started
Open AlphOBD
Click Bluetooth icon
Select the OBDLink MX+ adapter
Select Jeep, Grand Cherokee (WK2) MY2011>
Select Body Computer
Select Control Unit MY2014-2017 PowerNet
Connect (if not already connected)
Click open hood icon
Select procedure to Program Tire Size
Select tire or enter circumference in MM (Diameter in inches X 25.4 X Pi)
My tires are 255/70-18 with a diameter of 32.1"
32.1 X 25.4 X Pi = 2,561 MM
I entered 2561 for the size in MM
Click Start (should see process completed)
Cycle key to off
Cycle key to Run (do not step on brake, do not start engine)
You will get some errors
Select ABS
Select ABS Ate/Continental MY2014-2017 PowerNet
Connect
Click open hood icon
Select Procedure ABS Initialization
Click Start (should see process completed)
Click open hood again
Select Procedure Static ECU Initialization
Click Start (should see process completed)
Click open hood again
Select Procedure Clearing Rolls Information
Click Start (should see process completed)
Cycle key to off
Cycle key to Run (do not step on brake, do not start engine)
You will get some errors
Select ABS
Connect
Click the Engine icon
Clear the faults
Cycle key to off
Cycle key to Run (do not step on brake, do not start engine)
This time you should not get any errors

Check speedometer

If they don't match, calculate new tire circumference as:

Speed from GPS divided by speed on speedometer times the previous circumference used.

When my speedometer read 65 MPH, I was actually going 63 MPH. I guess my tires were slightly smaller than the calculation.

63 / 65 X 2561 = 2482 MM

I then repeated the Program Tire Size using 2482 (and all the other steps to initialize the ABS and clear the faults)
This time when I compared the Jeep speedometer to the SmartPhone GPS they matched perfectly!
 

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2014 GC Limited 4X4
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Thanks for the write-up! I'll be doing this as soon as I can get my suspension swapped and put the new wheels on.
 

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Discussion Starter #3
I think I understand why my first tire circumference was off. I measured the diameter of the tire off the vehicle. When the weight of the vehicle is on the tire, the bottom half compresses and makes the effective circumference less. Another way to calculate the circumference would be to measure from the ground to the center of the wheel. That is the effective radius. So them multiply the radius times two, then times Pi, and then times 25.4 to convert inches to millimeters.

Or just do you best estimate for the first attempt and then use the instructions above to refine the number and get it perfect on the second try.
 
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If you saw my previous post, I upgraded the tires and wheels on my JGC using a set of the 2019 Wrangler Sahara tires and wheels. After putting on the larger tires, my speedometer was no longer correct. I was able to correct it using AlphOBD. Here are the detailed instructions for anyone else that would like to do the same.

My setup
2014 Jeep Grand Cherokee Limited V6
Changed tires to 255/70-18 Bridgestone Dueller A/T
Purchased OBDLink MX+ and AlphaOBD (other adapters may work, check the AlphaOBD web site for a list of adapters)

How to best check if your speedometer is correct:

Compare your Jeep speedometer to a Speedometer app on your SmartPhone that uses the GPS. I mounted my phone to the dash using a vent mount so that the phone speedometer and the dash speedometer are side by side. The app I used was called GPS Speedometer and Odometer by COOL NIKS in the Google Play store. I liked the app, so I purchased the Pro version for $1.10.

Take your Jeep for a drive and find a long straight stretch of freeway where you can set the cruise control. The GPS speed is updated once per second, so the changes in speed lag behind the actual speedometer if you are speeding up or slowing down. That's why maintaining a constant speed is necessary to compare the Jeep speedometer to the GPS speedometer.

For me, when my Jeep speedometer read 65 MPH, I was actually going 68 MPH after putting on the larger tires.

Here is how to make the correction:

Setup OBDLink MX+ with OBDLink app to make sure the OBDLink adapter is working properly first

Connect to Jeep with ignition in Run, but engine not started
Open AlphOBD
Click Bluetooth icon
Select the OBDLink MX+ adapter
Select Jeep, Grand Cherokee (WK2) MY2011>
Select Body Computer
Select Control Unit MY2014-2017 PowerNet
Connect (if not already connected)
Click open hood icon
Select procedure to Program Tire Size
Select tire or enter circumference in MM (Diameter in inches X 25.4 X Pi)
My tires are 255/70-18 with a diameter of 32.1"
32.1 X 25.4 X Pi = 2,561 MM
I entered 2561 for the size in MM
Click Start (should see process completed)
Cycle key to off
Cycle key to Run (do not step on brake, do not start engine)
You will get some errors
Select ABS
Select ABS Ate/Continental MY2014-2017 PowerNet
Connect
Click open hood icon
Select Procedure ABS Initialization
Click Start (should see process completed)
Click open hood again
Select Procedure Static ECU Initialization
Click Start (should see process completed)
Click open hood again
Select Procedure Clearing Rolls Information
Click Start (should see process completed)
Cycle key to off
Cycle key to Run (do not step on brake, do not start engine)
You will get some errors
Select ABS
Connect
Click the Engine icon
Clear the faults
Cycle key to off
Cycle key to Run (do not step on brake, do not start engine)
This time you should not get any errors

Check speedometer

If they don't match, calculate new tire circumference as:

Speed from GPS divided by speed on speedometer times the previous circumference used.

When my speedometer read 65 MPH, I was actually going 63 MPH. I guess my tires were slightly smaller than the calculation.

63 / 65 X 2561 = 2482 MM

I then repeated the Program Tire Size using 2482 (and all the other steps to initialize the ABS and clear the faults)
This time when I compared the Jeep speedometer to the SmartPhone GPS they matched perfectly!
This is worthy of STICKY-ness! (as in this procedure should be made a sticky)
 
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