Speedometer wrong but OBD2 reader correct

[cav303]

2005 Wrangler Rubicon.
my speedometer is very wrong (looks like 10mph ish faster than I am going whenever going over 30mph).
BUT I read the computer via OBD2 port with a cheap reader and the speed is very close to my real speed.

The speedometer isn't making erratic moves like there is a wiring problem. Do I just pop out the pointer and adjust it down 10mph? It does not show that I am going 10mph when parked, it shows the correct 0.

 

[MichaelGH]

What size tires are you running?

Anything other than a stock height will throw the speedometer up or down, depending on tire size.

If other than a stock size tire, you’ll need to recalibrate your speedometer using the correct method (either a speedometer gear change or an add-on speedometer “healer”).

 

[DirtDawg]

Is a speedo recalibration device in play on yours? If so, the PCM might be reading the correct speed from the TC's speed sensor while the speedometer reading is off by your stated ~ 10mph. From my own experience, the PCM requires speed signals to avoid throwing codes (as mine did a few months ago).

I had changed from 35" to 33" tires on my own 2005 TJ Rubicon as well as removed the old SuperLift speedometer calibration device. That calibrator's wiring harness connector had been damaged at some point, resulting in loss of signal when the connector loosened and backed apart, along with the sensor signal. With a new SpeedoHealer calibrator teed into the speed sensor with the provided y-harness, I logged combo of speeds shown on the gauge cluster vs Garmin GPS actuals, then made the calibration adjustment per SpeedoHealer instructions. Got within .5 mph average error at various speeds on the first try and called it good.

Just for reference, showing the 241 t-case's speed sensor location:

 

[cav303]

What size tires are you running?

Anything other than a stock height will throw the speedometer up or down, depending on tire size.

If other than a stock size tire, you’ll need to recalibrate your speedometer using the correct method (either a speedometer gear change or an add-on speedometer “healer”).

everything has been modified but I have not done any of these mods so IDK.
BUT what would need to be modified if the PCM is showing the correct speed and the gauge cluster is not?

 

[cav303]

Is a speedo recalibration device in play on yours? If so, the PCM might be reading the correct speed from the TC's speed sensor while the speedometer reading is off by your stated ~ 10mph. From my own experience, the PCM requires speed signals to avoid throwing codes (as mine did a few months ago).

I had changed from 35" to 33" tires on my own 2005 TJ Rubicon as well as removed the old SuperLift speedometer calibration device. That calibrator's wiring harness connector had been damaged at some point, resulting in loss of signal when the connector loosened and backed apart, along with the sensor signal. With a new SpeedoHealer calibrator teed into the speed sensor with the provided y-harness, I logged combo of speeds shown on the gauge cluster vs Garmin GPS actuals, then made the calibration adjustment per SpeedoHealer instructions. Got within .5 mph average error at various speeds on the first try and called it good.

Just for reference, showing the 241 t-case's speed sensor location:

View attachment 654422

I will check but with 33" tires and the PCM reading correctly I assume there is a corrected sensor in there or somewhere!
What you are describing, would that not show incorrect in both the PCM and the gauge cluster?

 

[DirtDawg]

The TC-mounted speed sensor is a hall-effect type device which reads revolutions as defined by gearing and tire diameter. Also noting, the 241 TC uses that type of sensor vs a mechanical gear as on other TC versions. I believe that sensor sends speed signals correctly to the PCM but uncertain if that's independent of what the speedometer displays. Others here with better system knowledge might chime in on that aspect.

Since yours is a 2005 Rubicon it will have the 241 transfer case with speed sensor located as shown and since its modified, I suggest looking at the harness leading to that and seeing if there is an aftermarket y-harness physically in play. If so, look for the associated recalibration box - perhaps in the engine bay or behind the glove box door adjacent to the fuse box.

Here's another shot of mine, shown with Rubicon Express skid plate removed during transmission and TC fluids service. The sensor harness is shown tucked above and forward of the locker pumps support bracket.