Jump to content

Speedo Re-calibration??


Recommended Posts

I posted a similar thought on a thread in the Classifieds, but got curious about peoples' thoughts here:

 

Am I the only one that ignores the re-calibration of the speedo after re-gearing; resizing tires, etc., and just uses a GPS instead?

 

I first settled on this when I altered a number of factors on my 2002Tii, and didn't want to pay the big bux for the unobtanium BMW parts.

 

Maybe it only makes sense if, like me, 90+ % of your MJ miles are off-road, but I still do it with my Tii. Any decent GPS would work fine, and be a lot cheaper than speedo work.

Link to comment
Share on other sites

I posted a similar thought on a thread in the Classifieds, but got curious about peoples' thoughts here:

 

Am I the only one that ignores the re-calibration of the speedo after re-gearing; resizing tires, etc., and just uses a GPS instead?

 

I first settled on this when I altered a number of factors on my 2002Tii, and didn't want to pay the big bux for the unobtanium BMW parts.

 

Maybe it only makes sense if, like me, 90+ % of your MJ miles are off-road, but I still do it with my Tii. Any decent GPS would work fine, and be a lot cheaper than speedo work.

I plan I getting a new speedo gear. It is not expensive and an easy install. I went here -> http://gojeep.willyshotrod.com/HowtoSpeedoGears.htm

to find the correct tooth count, then I contacted BLHTAZ and had him order me the gear from the dealership.

Link to comment
Share on other sites

I posted a similar thought on a thread in the Classifieds, but got curious about peoples' thoughts here:

 

Am I the only one that ignores the re-calibration of the speedo after re-gearing; resizing tires, etc., and just uses a GPS instead?

 

I first settled on this when I altered a number of factors on my 2002Tii, and didn't want to pay the big bux for the unobtanium BMW parts.

 

Maybe it only makes sense if, like me, 90+ % of your MJ miles are off-road, but I still do it with my Tii. Any decent GPS would work fine, and be a lot cheaper than speedo work.

 

My TJ was about 15-25 mph off (15 @30mph and 25 @60mph)... it cost a total of about $20 for the right tooth speedo gear, and took about 5 minutes to swap... I don't know if it's the same deal with the MJ's but if that's it, I don't think it's a huge issue to resolve...

 

SW2

Link to comment
Share on other sites

I don't know if it's the same deal with the MJ's but if that's it, I don't think it's a huge issue to resolve...

It is, and as stated VERY easy to swap and relatively cheep as well. Not a huge issue...

 

Quite possibly the hardest part is locating the correct gear to purchase?

Link to comment
Share on other sites

Quite possibly the hardest part is locating the correct gear to purchase?

Could be. Their were also long shaft one's and short shaft ones based on what year you have. I think 87-93 are long shafts and than 93+ are the short shaft ones. The long shaft one's shafts are almost twice as long.

 

93-2001 use part number 520676xx replace the xx with the tooth count you want.

 

91-1993 us some funky part number that seem to not have a rhyme or reason to them. It starts out at 520681xx but than I think it is 51-62 based on what tooth count you want. 51 = 29 tooth than 62 = 40 tooth. You can do that math to figure out what's in between.

 

87-1991 use part number J32127xx replace the xx with the tooth count you want.

Link to comment
Share on other sites

I use my GPS to see how far off I'm buy, and just use the speedo after that.

 

I know when it read 65 I'm going 70. So if I stick to the speed limit, at speeds below 65 I'm off less than 5mph, at 65 I'm going 5 over. Should be close enough to be safe from tickets. This is stock gears, but 235/75 tires instead of 205/75.

 

If I'm going any bigger and/or regear I might switch speedo gears, but have no intention of doing so for now.

Link to comment
Share on other sites

Wish I could find a nice 2002 in these parts that don't have the rear shock towers shot. :cry:

 

Rust? What is this rust of which you speak? :brows:

 

One of the many great things about living in Kalifornistan. We get a bit of surface corrosion from living on the ocean, but it's rarely structural.

Link to comment
Share on other sites

Wish I could find a nice 2002 in these parts that don't have the rear shock towers shot. :cry:

 

Rust? What is this rust of which you speak? :brows:

 

One of the many great things about living in Kalifornistan. We get a bit of surface corrosion from living on the ocean, but it's rarely structural.

:agree: :agree:
Link to comment
Share on other sites

In addition to speedo gears being a LOT cheaper than GPSes, there's also the fact that many engine functions are controlled by input to the ECU (or, in newer vehicles, the PTCU) that includes reading the vehicle road speed and comparing it to engine RPM, vacuum, etc. If the speedo is off by 3 or 5 MPH at highway speeds it ain't the end of the world, but if the speedo is off by 20 or 25 MPH at highway speeds ... in essence, the ECU is being "tricked" and isn't operating the engine at optimum efficiency because it thinks you're going a lot slower than you really are. Doesn't matter much when you're trail crawling in first gear and low range, but on the highway you could be affecting the fuel mileage.

Link to comment
Share on other sites

In addition to speedo gears being a LOT cheaper than GPSes,.

 

Well I have a GPS anyway, so that's not really my issue. :brows:

 

 

there's also the fact that many engine functions are controlled by input to the ECU (or, in newer vehicles, the PTCU) that includes reading the vehicle road speed and comparing it to engine RPM, vacuum, etc. If the speedo is off by 3 or 5 MPH at highway speeds it ain't the end of the world, but if the speedo is off by 20 or 25 MPH at highway speeds ... in essence, the ECU is being "tricked" and isn't operating the engine at optimum efficiency because it thinks you're going a lot slower than you really are. Doesn't matter much when you're trail crawling in first gear and low range, but on the highway you could be affecting the fuel mileage.

 

Is that the case with the older speedos? Not sure how a cable driven speedo would produce such a signal.

 

I'm still pretty new to the whole MJ thing.

Link to comment
Share on other sites

Most of the long shaft gears are now gone. When I look them up...they automatically supercede in the computer to the newer style part number so you have to do like I did and simply swap the shaft out of your old gear in to the new one. It is simple and takes only 2 minutes to do.

 

The whole gear change process, including the shaft swap, will take 10-15 minutes. :cheers:

Link to comment
Share on other sites

there's also the fact that many engine functions are controlled by input to the ECU (or, in newer vehicles, the PTCU) that includes reading the vehicle road speed and comparing it to engine RPM, vacuum, etc. If the speedo is off by 3 or 5 MPH at highway speeds it ain't the end of the world, but if the speedo is off by 20 or 25 MPH at highway speeds ... in essence, the ECU is being "tricked" and isn't operating the engine at optimum efficiency because it thinks you're going a lot slower than you really are. Doesn't matter much when you're trail crawling in first gear and low range, but on the highway you could be affecting the fuel mileage.

 

Is that the case with the older speedos? Not sure how a cable driven speedo would produce such a signal.

 

I'm still pretty new to the whole MJ thing.

:doh:

 

You're right, of course. My theory remains valid, the ECU is still being folled regarding actual road speed. But you are correct, it's the CPS that tells the older 4.0L what the speed is, not a direct speedo gear output.

Link to comment
Share on other sites

there's also the fact that many engine functions are controlled by input to the ECU (or, in newer vehicles, the PTCU) that includes reading the vehicle road speed and comparing it to engine RPM, vacuum, etc. If the speedo is off by 3 or 5 MPH at highway speeds it ain't the end of the world, but if the speedo is off by 20 or 25 MPH at highway speeds ... in essence, the ECU is being "tricked" and isn't operating the engine at optimum efficiency because it thinks you're going a lot slower than you really are. Doesn't matter much when you're trail crawling in first gear and low range, but on the highway you could be affecting the fuel mileage.

 

Is that the case with the older speedos? Not sure how a cable driven speedo would produce such a signal.

 

I'm still pretty new to the whole MJ thing.

:doh:

 

You're right, of course. My theory remains valid, the ECU is still being folled regarding actual road speed. But you are correct, it's the CPS that tells the older 4.0L what the speed is, not a direct speedo gear output.

 

So then, if I understand correctly, once you've changed gearing or tire size on the older vehicles, there's no way to "recalibrate" so that the ECU is getting proper road speed info, right? The ECU is making "assumptions" of road speed based on the original specs of engine speed, stock gearing, and stock final drive?

Link to comment
Share on other sites

The input to the ECU does not change or need to be calibrated with changing of gears or tires. It is read off of the CPS which is not effected by such changes.

 

Then that suggests that the older ECUs do not have a road speed component to their calculations, right?

 

So going back to my original point, there would be no need to recalibrate the speedo on the older vehicles if you're happy using a GPS as an alternative.

Link to comment
Share on other sites

 

So going back to my original point, there would be no need to recalibrate the speedo on the older vehicles if you're happy using a GPS as an alternative.

Correct... The only thing being effected would be the number of miles on the odometer. ;)
Link to comment
Share on other sites

I apologize if I created confusion there. Yeah, on the Renix vehicles the ECU reads the CPS. What messed me up is that back when my '88 XJ was a couple of years old I inquired at a dealership about adding cruise control, and the service manager (who lied to me about other things, so I don't know why this stuck) told me it couldn't be done because there was no way to connect a cruise control to the "vehicle speed sensor."

 

I guess that's still stuck in a mental pigeon hole. And in a perverse way, it's true -- since there is no "vehicle speed sensor," there's no way to interface cruise control with it. Why the guy couldn't just tell me "It's a bunch of work and we don't want to be bothered." I don't know. He was just one of those people who found it easier to always lie. I was only at that dealership because the original dealership had been sold and the new owners were crooks. I quickly found that this other dealership wasn't a lot better and stopped going there, too.

Link to comment
Share on other sites

When I lived in Canada (87 through 93), Canadian Tire sold after market cruise control kits for $100. They were setup to work with mechanical speedo cables, the one to the speedo was rerouted to the cruise control and another one from the cruise went to the speedo. They worked off engine vacuum.

 

The newer ones are all digital and use a servo to move the throttle.

Link to comment
Share on other sites

So perhaps I'm just too used to dealing with more esoteric vehicles?

 

I have, in addition to the Tii, two mid-60s Lotus Elans, and a whole bunch of vintage motorcycles.

 

Maybe I need to re-calibrate my thinking when it comes to my MJ? :D

 

 

Time to hijack the thread :brows:

 

Will you please post some pics of the pair of Elans. I'm a fan of Lotus and would like to see them.

 

Thanks, and now back to your regularly scheduled thread.

 

Mike

Link to comment
Share on other sites

So perhaps I'm just too used to dealing with more esoteric vehicles?

 

I have, in addition to the Tii, two mid-60s Lotus Elans, and a whole bunch of vintage motorcycles.

 

Maybe I need to re-calibrate my thinking when it comes to my MJ? :D

 

 

Time to hijack the thread :brows:

 

Will you please post some pics of the pair of Elans. I'm a fan of Lotus and would like to see them.

 

Thanks, and now back to your regularly scheduled thread.

 

Mike

 

They're both project cars (one much more so than the other) and I don't have great pics, but the 66 Elan S2 S/E looks like this:

 

26696244_3DnhC-L.jpg

 

14753221_3XK6s-L.jpg

 

And you can kinda see the Tii in the background of this pic:

 

14753213_5ogX7-L.jpg

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

 Share

×
×
  • Create New...