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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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Reactions: jcaligp

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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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2014 Grand Cherokee Laredo
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Does it tell you what the current mm setting is? you could multiply it X 1.04, or whatever your error is, and get pretty close.
 

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Discussion Starter · #7 ·
Does it tell you what the current mm setting is? you could multiply it X 1.04, or whatever your error is, and get pretty close.
No, it doesn't tell you the current setting.
 

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2012 Grand Cherokee Overland 5.7L
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71 Posts
Does alphaobd work on 2012.
 

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Discussion Starter · #9 ·
Does alphaobd work on 2012.
It should work the same on any WK2 from 2011 to 2017. From 2018 on there is a firewall on the diagnostic port that requires some extra hardware. I have a 2014, so I am not familiar with the 2018+ security.
 
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2020 Jeep Grand Cherokee Trailhawk, ProCharged 5.7L Hemi
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531 Posts
I am older and more lazy when it come to doing the math. lol
I use the hell out of this website for calculating tire sizes. Wheel Offset Calculator.

Great for comparing current and new size tire differences, wheel spacer measurements, it will even give you the tires circumference in MM (or inches). and so much more.

@EmailGuy Great information, thanks for posting!
 

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Discussion Starter · #11 ·
I'm older too, but you cannot use the tire circumference. Because of the compression of the vehicle on the tire, the effective circumference will be less. I set the tire size using the circumference and my speedometer read too high. I had to use a lower value to get it dialed in.
 
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Reactions: IDoMy0wnRacing

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PSA - my 32.1" Sahara wheel/tires register only 2mph off at 75mph (actual 77mph) by GPS. ie. our speedo's are actually ~10% slower from factory, meaning you're accruing miles 10% faster that count towards your warranty and maybe even your MPG number. I've not verified actual MPG by the "old school" method or miles / gal, full fill to full fill.

My wife's 2014 Ford Escape was the same story. Replaced with bigger tires, she's 2 miles off at 80mph. Sounds to me a cheat / loop hole by OEM's on fleet MPG :unsure:
 

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Discussion Starter · #14 ·
PSA - my 32.1" Sahara wheel/tires register only 2mph off at 75mph (actual 77mph) by GPS. ie. our speedo's are actually ~10% slower from factory, meaning you're accruing miles 10% faster that count towards your warranty and maybe even your MPG number. I've not verified actual MPG by the "old school" method or miles / gal, full fill to full fill.

My wife's 2014 Ford Escape was the same story. Replaced with bigger tires, she's 2 miles off at 80mph. Sounds to me a cheat / loop hole by OEM's on fleet MPG :unsure:
My speedometer was reading high with the stock tires and low with the larger tires. With this process, my speedometer matches the GPS speedometer perfectly. I've even noticed that it matches the radar signs.
 

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Thanks for putting this together! About to up-size tires on my wife鈥檚 WK2 and will be glad to keep the speedo correct.
 

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I added Falken Wildpeak AT3Ws in 275/65R18 today and used the 2,482mm figure you solved for. Went for a test drive and speedo is spot on to the GPS @ 70mph. Thanks again for writing this up!
 

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I don't get it. I checked today by my bike app, nav and waze i'm spot on at 70 on cruise control after the switch to 32.1" 255/70R18 (sahara's) without any module or programming anything. 76mph GPS at 75 on cruise control. That meant my stock odo or cal was ~10% off. I can't imagine us having different hardware or software.
 

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Anyone get it to work with parksense/FCW? My 2018 will not correct the speedo but I get a parksense unavailable no matter what I do. Even clearing the parking module codes I get an error

Vehicle Line Mismatch B222A
 

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2020 Jeep Grand Cherokee Trailhawk, ProCharged 5.7L Hemi
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Anyone get it to work with parksense/FCW? My 2018 will not correct the speedo but I get a parksense unavailable no matter what I do. Even clearing the parking module codes I get an error

Vehicle Line Mismatch B222A
W E L C O M E ! ! !
to the forum.

IF you try to adjust your speedometer (for example) for larger tires. you will (I did) get a Parksense error, followed up by a Vehicle Line Mismatch B222A.

For some reason, the largest size that you can adjust to is: 2440mm. At 2440mm the Parksense and the Vehicle Line Mismatch B222A will go away.

Make sure after you adjust the tire size (to correct the speedometer) that you perform the ABS initialization procedure, and it needs to be performed in the following order:
Static ECU initialization ;
Longitudinal acceleration sensor;
Clearing Rolls information.

Speaking from experience on my 2020 Grand Cherokee Trailhawk, 5.7 Hemi.
I changed my tires from the oem 265/60R18 (original BCM tire setting of 2371mm) to a 275/55R20, Falken Wildpeak AT3W which is a 31.9 inch tire @ 2545mm circumference.

I have the Adaptive Cruise Control, Parksense, Lane Detection... and it seems that our Jeep does not like anything larger than a 31 inch tire.

At the tire setting of 2440mm with my 275/55R20 (31.9 in) my speedometer is off by LESS than .5 (1/2) mph at 60.
Speedometer reads 60 and GPS flickers between 60 and 61 mph.

I hope this helps.

Ed
 

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W E L C O M E ! ! !
to the forum.

IF you try to adjust your speedometer (for example) for larger tires. you will (I did) get a Parksense error, followed up by a Vehicle Line Mismatch B222A.

For some reason, the largest size that you can adjust to is: 2440mm. At 2440mm the Parksense and the Vehicle Line Mismatch B222A will go away.

Make sure after you adjust the tire size (to correct the speedometer) that you perform the ABS initialization procedure, and it needs to be performed in the following order:
Static ECU initialization ;
Longitudinal acceleration sensor;
Clearing Rolls information.

Speaking from experience on my 2020 Grand Cherokee Trailhawk, 5.7 Hemi.
I changed my tires from the oem 265/60R18 (original BCM tire setting of 2371mm) to a 275/55R20, Falken Wildpeak AT3W which is a 31.9 inch tire @ 2545mm circumference.

I have the Adaptive Cruise Control, Parksense, Lane Detection... and it seems that our Jeep does not like anything larger than a 31 inch tire.

At the tire setting of 2440mm with my 275/55R20 (31.9 in) my speedometer is off by LESS than .5 (1/2) mph at 60.
Speedometer reads 60 and GPS flickers between 60 and 61 mph.

I hope this helps.

Ed
Yeah I have tried every tire size imaginable and it goes away instantly with stock tire size. 2440 won't even work. I feel like mine is even different than a 2020/2021 because this isn't the first time I've heard that 2440 number which doesn't work with mine. I'm going to work closely with a local dealer to see if they can do something similar to the rams where the dealer must program something into the computer to allow larger tire size changes
 
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