• NAXJA is having its 18th annual March Membership Drive!!!
    Everyone who joins or renews during March will be entered into a drawing!
    More Information - Join/Renew
  • Welcome to the new NAXJA Forum! If your password does not work, please use "Forgot your password?" link on the log-in page. Please feel free to reach out to [email protected] if we can provide any assistance.

Speedo correction

Hes asking about changing the speedometer gear, if you have different gears and/or different then stock size tires your speedo will be off.

For the price of a speedo gear I doubt the dealership could flash the pcm anywhere near it. I also do not think they can reflash for the speedometer since ours is still gear driven, I could be wrong though.
 
Depending on what transfer case you got you should be able to replace the tiny plastic gear that is right after were the speedo cable hooks onto the transfer. You should be able to get one at the deal. You will need to know how many teeth you would need tho.
 
Speedo gears can be had new for around $50 shipped, and it takes a few minutes to remove, index, and replace them. I don't see the point in fooling with the PCM, and I don't understand why guys don't just buy new gears and be done with it. Maybe some gears are particularly rare? I just looked up the part number, and bought one online.
 
No offense Guys...

The nature of my question was purely knowledge based, and didn't have anything to do with how inexpensive it would be to either ignore the problem, or swap gears.

My question was based on the knowledge that all OBD2 models are not cable driven, but have a tach at the speedo gear. There is an aftermarket correction box available, but I was wondering if any folks with experience have seen a programing solution?

Whitexj99-You came the closest to understanding the question, and I'm not discounting it, just was hoping someone who works for a Dealer has approached the problem from a different direction :)

-Ron
 
The 96-04 PCM can not be edited to correct for tire/gear changes. Nor can aftermarket plug into the OBDII Port electronics correct for it. The Chryco PCM is rather primitive in most respects. The GM ECM of the same vintages can be edited for nearly anything... Which is one reason the Custom Car folks go with them.

The only solutions I know are:
1. Start a run on the hiway. Zero the trip odemeter at a mile marker, go at least 30 miles (the more the better) and compare the trip reading to reality. Do the math and then just account for it as you drive. In other words, live with it...

2. Install a different gear in the transfercase. This will get you close. Then, do step 1...

3. Use the electronics you can add to the vehicle to get it closer to correct. Then do step one.

4. Install a Supercharger so you have an excuse to buy an AEM F/IC8. Use the Frequency Table to dial in the Speedometer to a gnats behind.
 
Speedo gears can be had new for around $50 shipped, and it takes a few minutes to remove, index, and replace them. I don't see the point in fooling with the PCM, and I don't understand why guys don't just buy new gears and be done with it. Maybe some gears are particularly rare? I just looked up the part number, and bought one online.

^^This. Do a search for speedo gear. There's an old post on here somewhere that has a chart for the number of teeth you need based on your gear setup. I would be these are the same for NV242/231 transfer cases. I know for a fact that up to 96 models, the pcm can not be reprogrammed for wheel size and it sounds like from others that the new models can't be either. What are the specs of your rig?
 
No offense Guys...

The nature of my question was purely knowledge based, and didn't have anything to do with how inexpensive it would be to either ignore the problem, or swap gears.

My question was based on the knowledge that all OBD2 models are not cable driven, but have a tach at the speedo gear. There is an aftermarket correction box available, but I was wondering if any folks with experience have seen a programing solution?

Whitexj99-You came the closest to understanding the question, and I'm not discounting it, just was hoping someone who works for a Dealer has approached the problem from a different direction :)

-Ron


IIRC, the ECU in an XJ can't just be reprogammed. A speedo gear will do the same and its relatively easy to install.
 
The 96-04 PCM can not be edited to correct for tire/gear changes. Nor can aftermarket plug into the OBDII Port electronics correct for it. The Chryco PCM is rather primitive in most respects. The GM ECM of the same vintages can be edited for nearly anything... Which is one reason the Custom Car folks go with them.

Ah, so that's the issue.

That would narrow our choices to the gear change or Superlift Tru-Speed box which is insanely expensive compared to a gear.

As I stated, it was an intellectual exercise as it seems I have not seen anything on flashing the PCM to correct for tire size changes.

-Ron
 
Back
Top