Three Speed Measurements, only two match!

[Mudderoy]

Many of you know that I have an OBD II bluetooth unit and software on my Andriod phone so I can monitor various reading coming from the Jeep's computer.

This software has the ability to display how fast the Jeep's computer thinks I'm going and how fast I am going as measured from GPS signals.

It's a great feature and it allows me to see exactly how far off I am.

To my surprise I was about 2 mph off between measured electromechanical and via GPS. I explained by figuring the tires had changed diameter over time and really it wasn't going to be 100% accurate anyway, not as accurate as the GPS.

Then I installed the 33" tires (prior tires were 32"). I ordered and received the new speedo gear to go in my tcase, but I didn't install it until last night.

Comparing the speed reading between GPS and OBD II I am still off a couple miles per hour!

Stranger yet when I compare the in dash speedometer and the GPS it's almost identical!

So what am I seeing from the OBD II?!?!?!!?

To recap...

GPS (I'm assuming this is correct) matches exactly with the in dash speedometer, however it is about 2 mph faster than what the OBD II is reporting.

[4.3LXJ]

Tony, I am not convinced yet that the Android GPS is an accurate speedo yet. My phone is way off, not even close.

[xj4life2]

I would assume that the OBDII is correct. I have seen this before, you will never get right on the money due to variations in tire size, actual molding of the gear ect ect . I am speaking of the OBDII vs. the speedo , I don't know squat about the phone set up you guys have. :)

[Mudderoy]

Tony, I am not convinced yet that the Android GPS is an accurate speedo yet. My phone is way off, not even close.

I'd be inclined to agree with you but the GPS matches my in dash speedometer exactly! lol

I have another GPS in my Jeep, I'll run it on the way home today and compare the two GPS mph readings.


I would assume that the OBDII is correct. I have seen this before, you will never get right on the money due to variations in tire size, actual molding of the gear ect ect . I am speaking of the OBDII vs. the speedo , I don't know squat about the phone set up you guys have. :)

Yeah but the thing that confuses me is where is my speedometer getting it's information from if it isn't coming from the OBD II?!?!?!? Remember the speedometer matches the GPS.

[Mudderoy]

I forgot to update...

I measured the speed with TWO GPS receivers, one rounded to the nearest whole number then other shows tenths. Both GPS mph matched. The speedometer matched the GPS(s). The OBD II didn't match any of them. :sign0181:

[LizardRunner]

I'd go with the gps reading. the obdII is probably trying to compensate for to many perameters to actually be accurate. I know my two garmins are very accurate in calculating mph in the jeep, the van and on the bike. My bike speedo reads 10% low but the gps reading matched the reading of my trooper friend's radar gun when we went out to see just how fast the pumpkin would go. In that instance the radar said 142, the gps said 142.

[Mudderoy]

I'd go with the gps reading. the obdII is probably trying to compensate for to many perameters to actually be accurate. I know my two garmins are very accurate in calculating mph in the jeep, the van and on the bike. My bike speedo reads 10% low but the gps reading matched the reading of my trooper friend's radar gun when we went out to see just how fast the pumpkin would go. In that instance the radar said 142, the gps said 142.

Oh believe me I trust the GPS, but where is the speedometer getting it's reading if not from the OBD II?!?!?!!

[4.3LXJ]

good question Tony. Must be a gremlin in there somewhere.

[XJ4IV]

perhaps the OBD has not been calibrated to reflect the new speedo gearing and thus the computer is still measuring by your actual motor RPMs rather that the actual output shaft rotation...

[LizardRunner]

Um that would be that spedo gear you put in.

"Then I installed the 33" tires (prior tires were 32"). I ordered and received the new speedo gear to go in my tcase, but I didn't install it until last night."

[Mudderoy]

Um that would be that spedo gear you put in.

"Then I installed the 33" tires (prior tires were 32"). I ordered and received the new speedo gear to go in my tcase, but I didn't install it until last night."

lol I am not following or we're going in circles.

My assumption is there is ONE way for the Jeep to measure the speed. It is from the speedometer sensor that has a multi-tooth gear plugged into it and the whole shooting match inserted in a big freaking hole in the transfer case.

That would mean that the OBD II get's it's information from that sensor and by some MAGIC the speed is displayed on the analog dial in the dash.

With those assumptions, the analog dash, GPS and OBD II should all match!

If no one knows the answer we'll just assume I'm crazy and we'll move on.

They don't! GPS and the analog dash gauge do, the OBD II does NOT.

They didn't match with the prior speedo gear, they still don't match with the new speedo gear.

:rant:

[Mudderoy]

Through the process of trying to debug my torque converter lock up issues I have found (on the Internet I have yet to verify physically) my 97+ AW4 transmission as 2 TWO speed sensors. Input and Output.

This would explain why I show 2 different speeds, if the computer is reading and sending to the OBD II the input speed sensor, but the output speed sensor (that I have corrected for changes in gearing and tire size) is showing correctly on the speedometer.

This also may be why my torque converter isn't locking up if the input speed sensor has a problem.

[jeepxj95]

very possible