Speedo problems

[Master Gunner]

Just changed the gear in my trans to try to get my speedo close, but after going from a 38 to a 36, it still reads the same. Any ideas out there of anything I overlooked?

 

[rlem1977]

Maybe this will help if you haven't used one before:

http://www.tciauto.com/tc/speedometer-gear-calculator/

 

[Master Gunner]

Right, but what I'm saying is I changed gears from a 38 tooth to a 36 tooth and the speedo is off the same amount, how is this possible?

 

[rlem1977]

How far off do you believe your speedometer readings are? A rough rule of thumb for GM, don't know about Mopar, is approx. 3% to 5% change / tooth. That's only 3 to 5 mph difference at 100 mph.

 

[Master Gunner]

10 MPH @ 60 using GPS. I've used GPS to know it's not exact, but it's close, and for two gears showing the same MPH something's off.

 

[rlem1977]

10 MPH @ 60 using GPS. I've used GPS to know it's not exact, but it's close, and for two gears showing the same MPH something's off.

Could be internal to the speedo head.

 

[Master Gunner]

Never thought of that, hell, the speedo's only 44 years old, why should it need a rebuild? I think you got it, time to get the cluster re calibrated. Thanks for the help.

 

[AR67GTX]

Speedo heads have to be calibrated for the magnetic field. But it's just not likely that would cause the issue you are describing. Either the speedometer head works in which case it will register speed in a linear relationship to the cable speed - or the innards are worn and it will wag and jump all over the place. Change the driven gear in the transmission with one of different tooth count and the cable speed will change and the speedometer will register accordingly. If you are 10 mph off at 60 (or there abouts) you need to change more than 2 teeth.

 

[Cranky]

I don't wear them....but make sure of this....The gear housing must be placed in the correct orientation for the particular driven gear being used. The three positions broken down by tooth count are; 25-31, 32-38, 39-45. If the orientation isn't right, it could present a problem, but 2 teeth might not show much difference at 60 but I would think it would show something different at least. Another thing might be dirt in the head or the needle might be very slightly dragging. Kinda doubt it's the needle but possible. Do you have an area where you can get up to a higher speed? And some times a GPS isn't right on the money either. Heck, how do we know if the earth is spinning at a constant speed??

 

[Wietse]

The amount of difference might be too small to notice, including the deviation you have on the GPS.
Did you follow up on riem1977 his link? (or something similar) and did this tell you to install a 36 teeth gear?

 

[Master Gunner]

Here are the details, 3.91 gear, 28 inch tall tires, by the charts it should be the 37 tooth white gear. This was off about 5 MPH so made a mistake and went the wrong way and installed the 38 tooth blue gear which gave a 10 MPH error. I verified this with GPS and the old stopwatch and mile marker test @ 60. Changed gear again to the red 36 tooth gear and still 10 MPH off. What gives?

 

[Wietse]

Ok, got the whole story now.
Think that being out 5MPH is not far out, mind these speedo's were not 100% (even modern cars show more then actual speed)
You could try a 35T gear, but it might give you a 5-10MPH reading the other way.

The other thing, as Cranky mentioned is the adapter orientation. (i don't wear them either )
You could try to turn the adapter a bit more towards the 25-31 position (CW direction), this will bring the speedo gear a bit closer to the drive shaft.
Just make sure you do not force it tight as this will cause damage to the plastic gear. (you might want to stick to it in the end being closest)
In that case you can't install the securing clip as the grooves will not line up, but maybe something else can be found as a temporary fix.
In the end the diameter are all a bit different and this size might be operating with just a bit to much gear clearance causing it to skip a few teeth and lowers the rotations on the speedo cable.

 

[hunt2elk]

A one tooth change is usually 1-2 mph at 60. I have had a couple of newer made ones not read the same as an old original also. Maybe this is your issue if you bought a current production one?