Speedometer Inaccurate and Bounces

[kdml]

The speedo on my '83 is not accurate and it bounces at lower speeds (below 20). I lubed the cable, but it still bounces. I was thinking of replacing the speedo to see if it will correct both issues.

I was also thinking that the gear on the end of the speedometer cable might be wrong and causing the inaccurate readings, however, the amount by which the speedo is inaccurate changes based on my speed. For example, at 25 the speedo reads 4 mph too slow, but at 55 it reads 10mph too slow. Would this be an issue with the cable/gear of is there a problem with the speedo itself?

Thanks
Doug

[geezer#99]

Change the cable first and then see how accurate it is.

[Malo83]

I have a similar problem with my 83, during high speed driving on the highway after about 30 minutes the needle start going all over the place, works normal around town

[Chevy1966]

Originally Posted by kdml

The speedo on my '83 is not accurate and it bounces at lower speeds (below 20). I lubed the cable, but it still bounces. I was thinking of replacing the speedo to see if it will correct both issues.

I was also thinking that the gear on the end of the speedometer cable might be wrong and causing the inaccurate readings, however, the amount by which the speedo is inaccurate changes based on my speed. For example, at 25 the speedo reads 4 mph too slow, but at 55 it reads 10mph too slow. Would this be an issue with the cable/gear of is there a problem with the speedo itself?

Thanks
Doug

Doug, most likely you have a couple of things going on here and I have encountered all of them in different trucks that I have owned. First as far as the bouncing is concerned it is 90% likely that the speedo cable is to blame. If the inner jacket of the cable is worn, then no matter how much you lube it, the cable core can still bounce around. Less likely is that the bearings inside the speedometer are sticky or worn. Replace the cable first, that is my suggestion too.

Once you have the bouncing problem solved you can address the accuracy issue. There are several factors that affect the speedometer's actual reading: the plastic speedometer gears in the transmission (you have a "drive" gear and a "driven" gear), the differential ratio, and your tire size (outside diameter). Take a look at the following page for some good information and pictures explaining what this means:

http://www.transmissioncenter.net/sp...n_______va.htm

Here is a related link with a little calculator for finding the correct gear sizes:

http://www.bgsoflex.com/speedo1.html

The above links will give you a lot of help in understanding the details of how the speedometer gets the correct input for determining your speed. Of course it is possible that the speedo itself is out of calibration, but you should have all of the other components correct before you come to this conclusion. Good luck - Rob

[TBONE1964]

If you want to send me your speedometer head, I can check it for calibration and needle bounce.

More than likely, if you have needle bounce at a low speed (below 25 MPH), it is caused from worn drive bushings in the speedometer frame or the cable. If it bounces at a higher speed (over 25) then the problem is most likely a weak magnet in the speedometer head.

Let me know if I can help you with you issue. If you have a speedometer shop local to you, they can check your speedometer head for calibration accuracy and needle bounce.

Take care,
Tom

[kdml]

New speedo cable has been installed and speedo bounce is gone; however, speedo is still inaccurate. Will need to pull inspect speedo gears to see what I have. I know gear ratio is 3.42 and tire size is 265/75-16, which comes out to about 31.5"

Thanks

[dhughins]

I have similar problem on my 84. What speedo cable did u get?

[TBONE1964]

Great! I always like to here follow ups when problems get resolved. It is good information for others with the same issue.

Glad you got it fixed,
Take care,
Tom

[kdml]

Originally Posted by dhughins

I have similar problem on my 84. What speedo cable did u get?

Picked up cable at local Autozone - here's a link

http://www.autozone.com/autozone/par...r=1667_0_7079_

[kdml]

Finally, had a chance to look at the speedometer gears in tailshaft.

The driven gear was green and had 42 teeth.

I couldn't get my head up next to the hole in the tailshaft to see the drive gear, but I took stuck my phone up next to it and took a picture. It was blue, so that would mean its a 18 tooth gear, based on the chart in the in the above link to PATC. However, it doesn't look like any of the drive gears in the link. The ones in the link show the gears are nothing but teeth and are narrow. Mine is much wider. Are the drive gears for a 4wd trucks different? It looks a lot like the Dodge NP208 drive gear in the link (part #208D), which is listed as having 13 teeth. Any thoughts on which drive gear I have?

Thanks
Doug

[dfrank]

Originally Posted by kdml

Finally, had a chance to look at the speedometer gears in tailshaft.

The driven gear was green and had 42 teeth.

I couldn't get my head up next to the hole in the tailshaft to see the drive gear, but I took stuck my phone up next to it and took a picture. It was blue, so that would mean its a 18 tooth gear, based on the chart in the in the above link to PATC. However, it doesn't look like any of the drive gears in the link. The ones in the link show the gears are nothing but teeth and are narrow. Mine is much wider. Are the drive gears for a 4wd trucks different? It looks a lot like the Dodge NP208 drive gear in the link (part #208D), which is listed as having 13 teeth. Any thoughts on which drive gear I have?

Thanks
Doug

Yes the 4wd gears are differnet since they are in the transfer case and not the transmission. Mine looks exactly the same. I can't remeber how many teeth it has so sorry I can't help you there.

[kdml]

Finally received my new driven gear to address my accuracy issue. I replaced the existing 42 tooth gear with a 39 tooth gear. The 39 tooth gear is based on the 18 tooth drive gear, 3.42 differential gears and 31.5" tire diameter.

Now when I am doing 55, the speedometer reads 70. So now I am reading too fast. Do I need to try a 40 and 41 tooth driven gear or do you think my problem is the speedometer?

[Axle]

If you want to slow down the speedometer, you need MORE teeth on the driven gear.

With your original driven gear, what was your actual and indicated road speed?

Alex.

[GRADYS Performance]

Mark and Count the teeth on the Drive Gear. Do not rely on color.

[kdml]

I picked up a used/re-calibrated from Tom (TBONE1964) and installed this past weekend. With the new speedometer and change in driven gear everything is right on. Really good to finally know how fast I am going

Thanks

[TBONE1964]

Originally Posted by kdml

I picked up a used/re-calibrated from Tom (TBONE1964) and installed this past weekend. With the new speedometer and change in driven gear everything is right on. Really good to finally know how fast I am going

Thanks

Very good. Glad you have it all working as it should.

Happy to help.

Take care,
Tom

[craig113]

My needle bounced while the vehicle was under warranty. I watched the mechanic at the dealership pull the cable out of the housing and took it to a grinder and shortened it 1/8 inch, lubed and reinstalled it. All was good. Found out over time the cable will stretch when it starts to dry out.

[kdml]

With the original 42 tooth driven gear the speedometer read too slow, which is why I purchased a driven gear with less teeth. At 55 the speedo read 45.

And I did not rely on the color of the drive gear, I marked it and counted the teeth, by jacking up the rear of the truck and spinning the driveshaft.

I will do the odometer test before proceeding any further

Thanks

[kdml]

Ran the odometer test today. It was right on, so it appears my issue is with the speedometer

[Chevy1966]

You can't go wrong with the advice from TBONE... this is his business and it was a great suggestion to use the odometer as a test reference.

[TBONE1964]

Originally Posted by Chevy1966

You can't go wrong with the advice from TBONE... this is his business and it was a great suggestion to use the odometer as a test reference.

Thank you for your kind comment. You cannot go wrong doing the 1 mile odometer test. It will tell you if for sure if your transmission gears for the speedometer are off or if the speedometer itself is out of calibration.

Glad it is figured out.

Take care,
Tom