Speedo Discrepancy

Find other things that are actually important to be OCD about. ;)

It’s important to the OP.

Is it a small thing, yes but it matters to someone. I don’t care if my Speedo is off but it annoys the f out of if my bolts/bolt heads don’t match.

To each their own.


OP
If you tweak the actual signal to correct the speedo then your actual speed will be off. You’ll need to pull the cluster, pluck the needle off and set it to where you feel it’s correct.
 
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It’s important to the OP.

Is it a small thing, yes but it matters to someone. I don’t care if my Speedo is off but it annoys the f out of if my bolts/bolt heads don’t match.

To each their own.


OP
If you tweak the actual signal to correct the speedo then your actual speed will be off. You’ll need to pull the cluster, pluck the needle off and set it to where you feel it’s correct.

Good point. Different things matter to different people to different degrees I suppose. :unsure:
 
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Ignore it and drive, why sweat the small stuff.

Mine is off by 5mph and I just adjust my driving habits.

If I can't get that closer than 5 mph, then why try to get anything else accurate? Less torque, lower grade bolts, less welding, less weld, shitty tires, where does it stop? Or put another way, if I care enough to get other stuff as correct as I can make it, why stop at the speedo?
 
I agree with this assessment. On 100% of the vehicles I've checked with a GPS, probably two dozen, the speedo reads dead accurate or higher, but never lower. A manufacturer can't get sued for a speedo which gets you a speeding ticket if the speedo is always off in the "safe" direction.

I believe the US standard for speedometer accuracy has words to that effect, but since those standards seem to live behind a paywall, I can't verify that. SAE J2976_201602 and SAE J2976_202205

I've adjusted at least 25 of them with a digital correction box so that the speedo is dead on with GPS.
 
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I've adjusted at least 25 of them with a digital correction box so that the speedo is dead on with GPS.

Speedo can definitely be dead on with GPS no problem. I think the reason for this post is that while his speedo gauge doesn't match GPS (due to the way he adjusted it and his choice of gear tooth count), the OBD speedo DOES match GPS. Which is actually pretty typical for a lot of vehicles and possibly the TJ too (though many might not realize it).

I'd be interested to see you run OBD on one of the ones you've corrected and see if the OBD is a bit slow when the gauge is accurate. Same for odometer count.
 
Jet's website says it is only available for automatic transmissions 🤦‍♂️

A nugget of info I related to their engineering department and here we are 5 years later, still not corrected. The transfer case does not know what transmission is in front of it, has no way to know, nor does it matter.
 
A nugget of info I related to their engineering department and here we are 5 years later, still not corrected. The transfer case does not know what transmission is in front of it, has no way to know, nor does it matter.

Does it go between the plug and the speed gear on the transfer case on the TJ?
 
Don't know, don't care. All I care about is can I get the speedo to match the GPS and I can.

Well, of course, and I do the same. All I'm getting at is that if the OP does what we do (and everyone else for that matter) and further corrects the speedo, whether via the JET or a gear with another tooth or two, then the Torque OBD app he references will be off. Not that that matters to most, but it will be. And that was the discrepancy between the two that he cared enough about to make a post for.

I wouldn't personally care about the OBD myself either, but that was what the post was concerned about, was a discrepancy between the two. Had he corrected the speedo to completely accurate on the first round, then he'd probably be posting asking why the OBD speed is off.
 
Does it go between the plug and the speed gear on the transfer case on the TJ?

Yes, but techically it has an in and an out. The in connects to the output signal from the speed sender, that goes into the box, the corrected signal goes out to the PCM. We wire them in by the PCM on the firewall.
 
Is the OBD 2 speed data accurate to the GPS throughout the entire range of speed? If so you may be able to simply pull the needle off the speedometer and put it back on at a fixed speed. Clusters are assembled separate of the vehicle with a tolerance. They sometimes are off by a bit. I have done this to other Chrysler vehicles that the cluster did not match the OBD and it works.
 
Is the OBD 2 speed data accurate to the GPS throughout the entire range of speed? If so you may be able to simply pull the needle off the speedometer and put it back on at a fixed speed. Clusters are assembled separate of the vehicle with a tolerance. They sometimes are off by a bit. I have done this to other Chrysler vehicles that the cluster did not match the OBD and it works.

I tried to say that in post #22
 
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If I can't get that closer than 5 mph, then why try to get anything else accurate? Less torque, lower grade bolts, less welding, less weld, shitty tires, where does it stop? Or put another way, if I care enough to get other stuff as correct as I can make it, why stop at the speedo?

Because the speedometer is the least of my worries.

Torque, bolts, welds, tires are all a safety concern.

My speedometer, once I know how much it's off, I can adjust my speed accordingly.
 
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Because the speedometer is the least of my worries.

Torque, bolts, welds, tires are all a safety concern.
You can work around lesser grade bolts, just take it easy and don't work the rig as hard. No biggy.
My speedometer, once I know how much it's off, I can adjust my speed accordingly.
And once you know your hardware is subpar, you can just adjust accordingly.
 
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