2nd Gen Non-Engine/Transmission speedometer calibration

[SBradshaw]

Alright guys... . I 've searched the archives but have not found the answer. . so I need your help. I have a 97 3500 4x4 I went from a stock 215 85 r16 tire to 235 85 r16 and of course the speedometer and odometer are off. I have talked to the stealer and they can get the gear to correct the problem but I have to tell them how many teeth I need on the gear. I would assume that I need a gear with less teeth than stock but I don't know how many. Does anyone know how many teeth are on a stock gear? Or better yet what gear I need? I know the speedometer is off only a little but I would still like to correct the problem. Any help would be great.

 

[parcher]

You're gonna have to do some math. You need to figure out how far off your speedo is, this will not be a fixed value it will be a percentage. Try and have someone "pace" you at 50mph if your speedo reads 47mph you are off by 6% and will need a gear with 6% fewer teeth than the one that's in the truck, you're going to have to take yours out and see what's in there. There is also a formula to figure out vehicle speed if you know engine rpm and final drive ratio. The formula is in a book called "Auto Math" by HP books.

 

[Dennis Garrett]

It's easy to regear, I think I went from 32 to 31 tooth gear the first time. Anyway, you have to decide which you want to be the most accurate, the odometer or the speeedometer. (You will only get one or the other, one will still be either fast or slow) I wanted both to be right, so I regeared to the the odo as close as I could get it, then re-positioned the speedometer needle on it's post to indicate true speed. Pull the trim, take off the plastic instrument panel lense. Carefully, & firmly pull the speedo needle off it's post. Drive down the highway at 60 or 70 mph, and determine your exact speed by GPS unit or stopwatch. Then without changing your speed, carefully press the speedo needle onto it's shaft at the correct speed you're travelling. (Trim pops right off after removing ash tray & cup holder. Lense has a few phillips screws).

 

[vssman]

On 12V 2nd gen trucks - Replace the VSS gear with a new one that has the correct number of teeth. The VSS unit should be located on the driver's side of the transfer case. It has a three wire connector attached to it. You need a 10mm socket. If you pull the VSS adapter out of the transfer case without twisting it you won't have to replace the o-ring. The gear will snap off/on the adapter. You may have to rotate the adapter to a new location depending on what gear your changing from/to. Also, very little fluid will spill out so you don't have to worry about the levels. Replacing the gear will correct the speedometer, odometer and have the TC lock at the correct time. Any competent dealer parts dept should be able to help you out with the number of teeth you need. You can also call Rickson wheels - They are more than willing to help too.

 

[onestackdram]

Ok what about those of us with the nv 4500 and 2 wheel drive that need to do the same thing? I went from 215's to 255's on my 3500. By my mathmatical skills I am off about 5%. So does that rule still aply to the number of teeth needed? And further more where is the gear located on a 2 wheel drive?

 

[parcher]

I don't know when it happened, but at some point Dodge took the gear driven vehicle speed sensor out of the BR truck and started calculating vehicle speed by counting the teeth on the rear ABS sensor (mounted on fear differential carrier) and "knowing" tire size. This is called "pinion factor" There are only a few tire sizes to choose from in the computer (can't recall if it's the CAB or the PCM). If your tire size is not in the computer you are S. O. L. The tire manufacturer will have a chart with a spec called "rollout", find a tire in the computer with this spec as close to the one on your truck as possible, this is the closest you will get.

Dennis Garret, I beleive you are confused. The proceedure you describe to calibrate your speedo only works at that particular speed. If you don't have the correct pinion factor or drive gear you will ALWAYS be off by a fixed percentage. Your speedo and odometer use THE SAME vehicle speed signal whether analog od digital. The user cannot calibrate one without affecting the other in a like manner.

I'm DaimlerChrysler GOLD certified, an ASE master automobile technician and have a AOS degree in automechanics, thrust me on this.

 

[Dennis Garrett]

NO, I'm not confused. When I got my truck new in late 96, the odo was fast by 7% and the speedo was 6 mph fast! I went to three dealers on the 200 mile trip home, and nobody knew how to fix it. One said it had a speed sensor going bad, they would have to change out the instrument cluster, send it in for "re-calibration" & several other "guesses". One said it was within Chrysler specs! Finally I crawled under it, and pulled the plastic gear, and purchased another one at the third dealer for $12. 00. It was a tooth bigger, I think 32 instead of 31, and that slowed the readings, but the odo now said 9. 85 in 10 miles, and the speeedo said 77. 5 at 75. Later I put on 265 tires inplace of the 245's, and both odo & speedo were too slow then. I regeared again and got the odometer very close for the bigger tires but the speedometer was off now. , worse that before. That's when I read a post here about repositioning the speedo needle, and that's what I did to get it right. And by GPS, it's very accurate now at ALL speeds, 30, 60, or 90. In 98 they changed the speedo drive somehow, maybe like what you're saying, but this worked for my 97, and will probably be the same for 96 & 95. By the way, how could something this simple totally stump three "dealers"? I sent in an article to TDR about this, you can read it in issue 17, summer of 97,pages 34-35. A couple other articles there about speedo error too.

 

[Berrigan]

PARCHER,



How do you change the computer to be accurate on trucks that use the ring gear sensor. Is it something we can do at home or is it a dealer only fix? I went to taller tires and both speedo and odo are reading 5% slow.



I crawled under the truck and found no old style gear drive on the transfer case. Glad I saw this post Bill

 

[Mundgyver]

For the electronic crowd, look up Abbot Electronic Ratio Control



http://www.atrol.com/cx_era.htm#era



This is what you will need. They work real nice.

 

[parcher]

Within the confines of the stock computer driven speedo set-up the end user cannot set pinion factor. BTW if you speedo and odo are 5% slow, you will have travelled 105,000 miles on your 100,000 engine warrany... ... ... ... hmmmm!

 

[Berrigan]

What would the dealer charge to reset it?



And ya you get an extra 5k on warranty



And I have to add 5% to my mpg.



Thanks Bill

 

[onestackdram]

Ok I ask again

Ok what about those of us with the nv 4500 and 2 wheel drive that need to do the same thing? I went from 215's to 255's on my 3500. By my mathmatical skills I am off about 5%. So does that rule still aply to the number of teeth needed? And further more where is the gear located on a 2 wheel drive?

 

[emjay]

MATH

The error introduced by changing tires converted to percent can be used to scale any other parameter, i. e. speedo gear teeth, ring/pinion, etc. :



NT=OT*(OTD/NTD)



Where:



NT = New number of teeth on driven speedo gear

OT = Old number of teeth on driven speedo gear

NTD = New tire diameter

OTD = Old tire diameter



The problem in this case is that you're stuck with one tooth increments. One tooth out of 33 is about 3%.



Moving the speedometer needle relative to the speedometer needle shaft introduces an offset, and does not calibrate the speedometer.



Another more accurate way to check the speedometer is to drive a predetermined route of at least 10 miles in a known-good vehicle, then repeat the exact route with the test vehicle and compare distances. Since most trip odometers have 0. 1 mile increments, 10 miles will give you a +/- 1% reading.

If the distance traveled is equal, and your speedo is still off, the problem is in the speedo itself, not the speedo gearing.

 

[Dennis Garrett]

I think the problem with mine from the start was the speedo needle being pressed on to the shaft at the wrong location, indicating incorrect speed, and it was out of correspondance with the odometer. When I regeared to compensate for incorrectness to start with, then later for larger tires, I got the odo very close, reading about 10. 15 for ten miles traveled, but the speedometer was 5-6 mph off at the same time. Moving the needle back to the correct speed while moving solved that problem on mine. Just today I put 550 miles on the truck on I-29 & I-35, and had my Garmin 76S keeping track of speed & distance. (mile markers are notorioulsly inaccurate) At an indicated 30 mph, the GPS read 28. 8, at 60, the GPS read 58. 8, and at 80, the GPS read 78. 8. Almost exactly the same degree of error at all speeds, which is probably due to 30k of tire wear since I repositioned the needle to true speed last year. After reading the above posts, I would have to say the needle on mine was never in the right place on it's shaft to start with. This worked fine for my truck, or I would not have posted it here. The idea actually came from another member who had done this on his vehicle. Someone posted later, there is a difference in speedo drive starting in 98, even the early 12v ones. Mine's a 97. I like to know exactly how fast I'm going, and nobody at Dodge/Chrysler could do anything for me, even under warranty. If they had send the speedo in to be "calibrated" they probably would just have done the same thing, reposition the needle to indicate correct speed. Hell, I can do that.

 

[Berrigan]

Originally posted by Dennis Garrett

I think the problem with mine from the start was the speedo needle being pressed on to the shaft at the wrong location, indicating incorrect speed, and it was out of correspondance with the odometer. When I regeared to compensate for incorrectness to start with, then later for larger tires, I got the odo very close, reading about 10. 15 for ten miles traveled, but the speedometer was 5-6 mph off at the same time. Moving the needle back to the correct speed while moving solved that problem on mine. Just today I put 550 miles on the truck on I-29 & I-35, and had my Garmin 76S keeping track of speed & distance. (mile markers are notorioulsly inaccurate) At an indicated 30 mph, the GPS read 28. 8, at 60, the GPS read 58. 8, and at 80, the GPS read 78. 8. Almost exactly the same degree of error at all speeds, which is probably due to 30k of tire wear since I repositioned the needle to true speed last year. After reading the above posts, I would have to say the needle on mine was never in the right place on it's shaft to start with. This worked fine for my truck, or I would not have posted it here. The idea actually came from another member who had done this on his vehicle. Someone posted later, there is a difference in speedo drive starting in 98, even the early 12v ones. Mine's a 97. I like to know exactly how fast I'm going, and nobody at Dodge/Chrysler could do anything for me, even under warranty. If they had send the speedo in to be "calibrated" they probably would just have done the same thing, reposition the needle to indicate correct speed. Hell, I can do that.

Ya ive driven the exact same route a year latter to the same destination and it was 2 miles farther on a 250 mile trip. Then the light went on. Tread wear makes the tire shorter. It has to spin more revs to go the same distance. I for one think 5% is too far off but would be happy with 1 or 2 %.

 

[Berrigan]

I just did the math and if a 31. 5 inch tall tire wore 1/4 inch of tread off, the speedo and odo would be off about . 75 percent.



So that means normal tire wear can throw you off one full percent.



Or check this am i wrong?



1/4 inch wear takes 1/2 inch off the overall tire height.

On a 31. 5 inch tire thats almost 2%.



Can tire wear error be up to 2%?





I think it can.

 

[emjay]

That is exactly correct. If you want to be a perfectionist, your speedo would be spot on at 1/2 tire tread wear, varying +/- 1 % throughout the life of the tire.