Speedometer delay

[Kai-by-Vecona]

Hello Everybody,

 

I have another riddle on my Dodge. The speedometer seems to show 30% off. If I'm driving 50 mph it only shows 35. Does anybody know if there's a possibility to adjust it?

Thank you in advance

 

Kai

 

[Robert Horne]

Maybe your speedometer only needs a few drops of lube.

[Kai-by-Vecona]

Hello Robert,

 

I installed a new speedometer cable and this works fine. Also on top of the speedometer is a little hole for greasing. I cleaned this well and spend some oil, but it still shows less speed. As my understanding is right, the speedometer, transfers the rpm of the cable in a electric signal that is shown. So the only idea I have is, that the parts which transfers the signal works not properly and I would like to know, if I could throw it into the trash or there is a possbilty to adjust it.

[Jim Yergin]

The drive gear on the end of your speedometer cable may not have the correct number of teeth to match your rear end axle ratio.

Sounds like you need one with fewer teeth.

Jim Yergin

[Kai-by-Vecona]

This could explain a lot. I go and have a look. Thank you Jim

[busycoupe]

You should use very light oil in the speedometer, only a few drops.  Since you have already oiled it I would not add any more.  As the oil soaks in the speedometer may work better.  The cable from the transmission spins a magnet in the speedometer.  The spinning magnet is surrounded by a metal cup attached to the needle.  The rotation of the cup and needle is controlled by a fine coil spring.  There is nothing to adjust inside the speedometer, and I strongly recommend that you do not try to take it apart.  There are many companies that will service old speedometers. 

As mentioned, you may have the incorrect number of teeth on the gear at the transmission end of the cable.  Another thing to consider is tire size.  If your tires have a larger diameter than the stock size this will cause the speedometer to read low.  Although tire size alone would not cause a 35% difference.

[_shel_ny]

Does the odometer read in error as well? Drive 100 miles, does it only show only a 70 mile change?

 

I ask that because I believe the odometer is gear driven- mechanically linked- where as the speedometer needle is as Busycoupe explained, magnetically driven.  If the odometer is not off by that same 30% it would indicate a problem in the speedometer, and not tire size/gear tooth induced error.

 

Those in the know------ please correct me if my thoughts of a gear driven odometer are incorrect

 

 

EDIT punctuation, grammer

Edited October 23, 2013 by shel_ny

[Don Coatney]

Right on Shel.

[TodFitch]

Shel is correct, the odometer is gear driven so if it is off then you need to check the gearing or tire size. They make devices to compensate that goe between the transmission and the cable if you change out the rear axle, etc. You should check this first and correct it before worrying about the speedometer reading.

 

If the odometer is correct then we are looking at the device busycoupe mentions: Basically the cable turns a bar magnet located inside an aluminum cup connected to the needle. As the magnet turns it creates eddy (electrical) currents in the aluminum which create an opposing force so the cup tries to spin with the bar magnet. The faster the magnet spins the more force is generated. A fine coil spring provides the return force and the balance between the spring and the magnetic force determines where the needle points.

 

Three things can go wrong with this:

1. The pivot point gets dirty or worn. In this case the needle can get stuck a some points or will read a bit off on the side you appoached the speed from. This does not sound like your problem.

2. The magnet can lose it strength. In this case the speedometer will read low. I understand that a good repair place can replace the magnet and/or re-magnetize the one that is there. This is not a "at home" repair unless you happen to have a good set of tools for working on things like watches.

3. The spring gets weaker with age. In this case the speedometer will read high.

 

For items 2 and 3, some speedometers are constructed so the fixed end of the spring is actually on a arm that is kept in place by friction. By moving the arm you can adjust how much return force there is and can compensate for some changes in the spring and/or magnet. However if the magnet is too weak then you'll find that if you adjust the instrument to read correctly at speed that it won't return to zero properly.

[Kai-by-Vecona]

hello everybody,

 

thank you very much for all these detailed informations. I have to check the odometer first. I never looked at it because we have Kilometers her in Germany and I have to calculate a special distance to see if the odometer works properly e.G. I have to look for a 16km distance and then I could look if the odometer shows 10 miles. If this is the case I think I would better look for some place the can repair the magnet stuff. I neither have the tools nor the experiences and I think it only could get worse.

[TodFitch]

 . . . I have to look for a 16km distance and then I could look if the odometer shows 10 miles. . .

Perhaps a GPS unit would be useful. The Garmin eTrex I use when hiking and camping as well as the mapping apps for my Android phone I use when driving both have the capability to display metric as well as English distance units. If you have access to such a unit, setting it to display miles for your odometer check might be easier than finding sign posted distances that work out for easy checking.

[Kai-by-Vecona]

Hello Tod,

 

this seems to be a good idea. I will look for an app for my iPhone. I think there will be something available. Thank you for the tip.

Kai

[Young Ed]

I use ulysses speedometer on android with good results.

[Merle Coggins]

I have Speed Box on my iPhone. It seems to work pretty good as a speedo.

[greg g]

Some times it is just a matter of dust or other contamination causing drag on the needle and other associated parts.  It causes enough drag on the driven piece to cause the reading to be off.  I believe some one a long time back said he some how got some low pressure compressed air into the speedometer housing and it was some what effective at dealing with a similar problem.

[hendo0601]

How tall are your rear tires? If they are shorter than the factory tires this will give you the incorrect speedometer readings you are describing here. I installed some shorter overall height tires on the back of my 66 chrysler newport and then took a 120 mile trip going 60MPH as indicated on the speedo...I was getting passed by everyone and their grandma, people were honking at me and giving me mean looks...and the trip took nearly an hour longer than it should have. Checking against a GPS speedometer showed that at 60 MPH indicated I was really only going about 47. Check the overall height of your rear tires against what came from the factory...they make speedometer cable gears with different numbers of teeth to correct for this.

[Kai-by-Vecona]

Hello Everybody,

 

was on a trade fair the last 5 days so I couldn't check.

 

Tire size is the original 6.00x16 so it might be something different. I downloaded an app for my iphone and I#m going to check Speedometer and odometer asap. I found in another topic the sometimes the gear for the odometers stucks to the needle, so the odometers turns wild and the needle get's too slow. A short time ago I wondered that the odometers shows lot's of miles since I bought the car but I didn#t drive very far. Perhaps there is a connection between these to things. I let you know what will happen.

 

Thank you in advance

 

Kai

[Kai-by-Vecona]

Hello all,

 

ok, odometer is fine but speedometer has a 15 miles delay beginning at about 15 up to 65 mph. I opended the speedometer and found a little thing wich adjust the tension of the spring. I loose it a little bit and it seems to be much better, but as TodFitch sayed, no the needle doesn't go back to zero it stops at 15.

 

I think, it might be the problem with the weak magnet but for the moment, I will keep it as is.

Thanks for your help

[martybose]

If the needle won't go back to zero you have a mechanical problem in the speedo that is preventing it from reaching zero.  From your description, it almost sounds like the needle is loose and shifted on the pin.  The magnet wouldn't have this effect.

 

Marty

[Kai-by-Vecona]

Hello Marty,

 

as I can see, the needle is fix. When I make the returns spring a little bit harder, the needle goes to zero but this results in showing too low speed. If I loose tension speed is correct but needle won't return to zero.

[Niel Hoback]

  
Possibly you need a little oil on the needle shaft bearing points.   Less than a drop of sewing machine oil should do it.

[Kai-by-Vecona]

I did that when I adjusted the spring. The mechanic looks great. I can't explain. I thought about making a new speedometer face that suits to the needle. Seems to be easier ;-)

[james curl]

Have you tried to oil the pivot for the needle? The old oil gets dust in it and thickens up and gets more resistance to rotation.  Since there is no mechanical connection between the needle and the cable, just the force field from the magnet, then any increased resistance results in a slower reading in the speedometer.

[Kai-by-Vecona]

Hello James,

 

yes I tried, but it had no effect. The needle turns very easy.

[bosworth]

Hi James,

I'm having the opposite problem with my speedometer, at low speeds it seems pretty steady and accurate but once you get up to 45 MPH or so all bets are off, the needle swings from 40 to 100, if the needle does steady out it reads 15 to 20 MPH over actual speed. After following this thread I am suspecting dust in the mechanism. Did you have much trouble pulling the head and opening it up?

Bill