Speedo

[Dobaroy]

Changed the rear end gears on the 77 Doba from 2.4 to 3.2 and trying to get the speedo working right (or close). Replaced the speedo gear from 24 teeth to 29 teeth as we have 255/70 R15 rubber on the back but we must be doing something wrong because we cannot get the speedo dialed in at all. Any help would be appreciated. Thanks

[AZ B-Body Fan]

I made the same gear ratio change on my 77 this year. I'm running 255/60/15 tires and used a 31 tooth gear. The current speed function of the GPS unit I used to check accuracy shows the speedo is accurate (I have not taken the time to check it with a stop watch or radar gun). Looking at one of the online speedo gear charts I used, the 29 tooth gear you chose is right, or at least real close. Wish I could be of more help.

Is it reading fast or slow? How far is it off? Didn't I read something about each tooth changing the reading by 2-3 MPH?

[hemisatellite]

I made the same gear ratio change on my 77 this year. I'm running 255/60/15 tires and used a 31 tooth gear. The current speed function of the GPS unit I used to check accuracy shows the speedo is accurate (I have not taken the time to check it with a stop watch or radar gun). Looking at one of the online speedo gear charts I used, the 29 tooth gear you chose is right, or at least real close. Wish I could be of more help.

Is it reading fast or slow? How far is it off? Didn't I read something about each tooth changing the reading by 2-3 MPH?

you are on it with the GPS, I went thru all that this year earlier repairing a speedo. That's why noone here can tell me what gear to put in the trans of my truck With 35" tall tires! I'm just gonna use my GPS and another car.

[DobaMark]

The best way to do this is to drive a measured distance, say 10 miles, and compare the odometer reading vs. the actual difference. Use these numbers to get a percentage error and multiply that times the current number of teeth to see how many teeth you need to add or remove for the hew gear. It's much more accurate than the charts of ratio and tire size. Once the odometer is accurate, then the speedometer is usually fairly close.

[Dobaroy]

Thanks for the replies. AZ, did you change the speedo gear adaptor when you switched to the 31 tooth gear? That is what we are waiting for the the mail now. Fact is our speedo is not working at all right now.

[R-body_mopar]

Changed the rear end gears on the 77 Doba from 2.4 to 3.2 and trying to get the speedo working right (or close). Replaced the speedo gear from 24 teeth to 29 teeth as we have 255/70 R15 rubber on the back but we must be doing something wrong because we cannot get the speedo dialed in at all. Any help would be appreciated. Thanks

Dobaroy,

The speedometer adapter for 21-25 teeth is #4028657, and the adapter for 26-39 teeth is #4058855; they are not interchangeable, as you will likely either have an inopperative speedometer or rip the teeth off of the gear by using the wrong adapter!! Don't ask how I know that......duh!

The parts manual calls for a 32 tooth black gear, #4058132, for tire size P225/70xR15 (the largest size tire listed) and 3.21/3.23 rear gear. You may need a 30 tooth yellow gear for the larger tire (there is NO 31 tooth gear listed)................?????

Confused YET??????

[Dobaroy]

Thanks R Body, the highest tooth we could get was a black 29 but we ordered a new adaptor a couple of days ago, will see if thats the problem.

[AZ B-Body Fan]

I did replace the adapter. I think the adapter has three positions and I installed it incorrectly the first time and the speedo did not work. You probably also have the gear that is all plastic? I had a difficult time finding one. Seems like the places that sell new gears do not have the higher tooth count. A friend who owns a transmission shop told me to check Fatsco.com, their number is 973-227-2487. They had the gear, but told me it was the last one in stock. If they don't have any more 31 tooth gears, they may have the 30 tooth gear. The price was only $25.00 plus shipping, I think that is what the new gears were going for. I found the staff at Fatsco to be very helpful.

[Dobaroy]

Thanks AZ the gear we have is a black plastic one and you are right it was hard to find. Thanks for the heads up on the Fatsco info.

[R-body_mopar]

Thanks R Body, the highest tooth we could get was a black 29 but we ordered a new adaptor a couple of days ago, will see if thats the problem.

I did replace the adapter. I think the adapter has three positions and I installed it incorrectly the first time and the speedo did not work. You probably also have the gear that is all plastic? I had a difficult time finding one. Seems like the places that sell new gears do not have the higher tooth count. A friend who owns a transmission shop told me to check Fatsco.com, their number is 973-227-2487. They had the gear, but told me it was the last one in stock. If they don't have any more 31 tooth gears, they may have the 30 tooth gear. The price was only $25.00 plus shipping, I think that is what the new gears were going for. I found the staff at Fatsco to be very helpful.

OK, thanks guys!

But...............since I have CRS disease, does anyone have a link to the different types of gears and adapters that Mopar used??

If I recall correctly, there were three different styles of gears over the years, and different adapters. The early ones, back into the '60's-'7?, used a metal adapter and a long shaft gear, but somewhere along the line in the mid-late '70's, the speedometer cable was changed from a screw-on end to a plug-in end, and the gear and adapter was changed to all plastic.
But it has been so long since I have worked on these.......I can't remember...............

It seems to me that this is a common problem here, because so many people have changed transmissions, rear end gears, and tire sizes over the years, and there are so many different styles of cable/gears that Mopar used over a period of time.

Can anyone simplify this, so we can put something in a reference archive?

How about DobaMark??......(you seem to have a handle on all the little Mopar tech changes.)

[Dobaroy]

OK this is what we have, a metal adapter with 3 different sets of numbers on it and we have a different metal adapter with only 1 set of numbers on it. Both of these take the screw on cable but I'm not sure which gear they use. Then we have 2 or 3 of the plastic adapters with only one set of numbers (21 to 25) and they take the small plastic gear. But then we also have a long plastic gear with the number 31 stamped on it. We are now waiting for the mailman to bring a plastic adapter with numbers 26-29 on it, as we have a 29 tooth small plastic gear. R-Body, did you mean that if you accidentally install it wrong that it would chew up the plastic gear?

[R-body_mopar]

OK this is what we have, a metal adapter with 3 different sets of numbers on it and we have a different metal adapter with only 1 set of numbers on it. Both of these take the screw on cable but I'm not sure which gear they use. Then we have 2 or 3 of the plastic adapters with only one set of numbers (21 to 25) and they take the small plastic gear. But then we also have a long plastic gear with the number 31 stamped on it. We are now waiting for the mailman to bring a plastic adapter with numbers 26-29 on it, as we have a 29 tooth small plastic gear. R-Body, did you mean that if you accidentally install it wrong that it would chew up the plastic gear?

This has been covered before on Moparstyle, but I cannot locate the threads; perhaps they have been purged.

I really cannot remember when they changed speedometer cables and gears and adapter housings, but I believe it was sometime in the mid-late '70's.

Roy.......do you have a FACTORY service manual?? The procedure for installing the cable and adapter is listed under Torqueflite-Transmission, "service in vehicle".

In my 1979 Factory manual, it is located on page 21-59, Figures #1 and 2.

If you have the plug-in cable, you will need the plastic adapter marked with the proper number for the number of teeth on the gear.....and this adapter is installed with the number at the 6 o'clock position, as they are offset. If you install it too close to the transmission shaft, you will damage the plastic teeth; too far away will result in the gear being misaligned, and not contacting the worm on the shaft. Also, be sure the housing and trans surface are clean and free of debris, and new O-rings are used, as they are prone to leakage, especially after being removed and re-installed!

Mopar used to make a package with ALL of the O-rings needed for replacement. (The number for the adapter O-ring is p/n 6031370.)

The long gear with 31 teeth and metal shaft is for the older style metal adapter using the screw-on cable. I believe they used both short and long metal shaft plastic gears in different applications, but I don't believe they will fit you plastic housings.


Hope this helps.........my memory is a little foggy on this!! Actually, it is a little foggy on a LOT of things..............LOL!

NOTE:

The part number for the plastic gears are all the same, except for the last two numbers, which indicate the number of teeth on the gear.

Example:

p/n 4058124 would have 24 teeth, and 4058129 would have 29 teeth, etc.

[DobaMark]

.
But it has been so long since I have worked on these.......I can't remember...............
....
How about DobaMark??......(you seem to have a handle on all the little Mopar tech changes.)

Same problem I'm having - I used to know the ins and outs of this back when I had cars I'd bought, one with the older and one with the newer setup, where the gears had been swapped (a 1967 Valiant and a 1982 Mirada). But my brain can't recall it. I guess it's the same one CRS syndrome.

All I can recall is once you get the parts you need, you index the adapter where it fits into the transmission by the # of gears on the pinion.

[Dobaroy]

Thanks guys, will let you know how we make out as soon as the adapter arrives.

[hemisatellite]

wasn't the old rule once that 0ne on tooth made up for 3 or 4 on the bottom end? granted yhe gear can be changed, but i also know about the diff adapters. best of luck