Speedometer - How it works and how to fix?

[Pino2234]

My 86 Ford F350 has 37s on it with 3.55s. The speedometer was way off and I wanted to stop using my gps to calculate fuel mileage bc now I have 2 trucks. Ford didn't make a driven gear for the speedo cable for my application because of my ratio and tire size. Its like i have overdrive with my t19. I currently have the 16 tooth driven gear (the lowest). I searched and found an 8 tooth drive gear for the transfercase that I installed. I think it had a 6 tooth drive gear before but I'm not sure.

So with my 16 tooth driven gear and 8 tooth drive gear I should be right on.

So I installed it and the tripometer seems to be on. In 29 miles the GPS says I went 29 miles and my truck says 29.2miles. But the needle is off. When the GPS says I'm doing 40mph my truck says I'm doing 45mph. And it's gets worse or better as I change speeds. It could vary from 1 - 10mph difference.

Now for my 2nd truck:
My 85 dually just has 33s on it and the speedo is way off as well. The speedometer is really slow compared to the GPS so you would think that the tripometer would be slow as well. But it's not. If I go 200miles on my GPS, my tripometer reads around 230miles even though the speedometer on the truck always reads slower than the GPS.

Also does anyone know the relationship between the speedo cable, the needle on the speedometer, the tripometer, and the odometer?

I'm just trying to get both trucks right on.

 

[Michael Fowler]

Generally speaking, speedometers have a cup that surrounds the pivot for the needle. The cup has magnets, and is driven by the speedometer cable. As the cup rotates, the magnets set up an eddy current which "pulls" the pivot ( and needle). The pivot for the needle has a spring to return it to zero. So the indicated speed is a balance between the magnetic forces, the spring, and friction. Too much lubrication, old, dirty lubrication, too little lubrication all can cause excessive friction and low readings.
Since your odometer part is indicating properly, I'd guess there is too much drag or friction for the needle to indicate the proper speed.
You can hit the junkyard for another speedometer, try and clean yours, or send it out to a specialty shop that does nothing but speedometer repair.

 

[Pino2234]

I never knew there were shops that repaired speedometers.

I tried finding speedometers at junkyards. The thing is I want a tripometer and I think that was started in 86 so I only have 1 year span to look.

Does anyone known of speedometer from 80-96 will work? I know they would fit in my dash but will they read correctly?

In my 86 being .2 off from the gps on the speedometer, do u thing that is the right speedo driven gear or 1 tooth up?

 

[tenlug]

Since the odo is close I would pull the speedo and take it apart. Lube the brass pin inside the bell and clean out the bell. You've got nothing to lose. Speedo's from 80 thur 86 will work. They are't that hard to find around here with a trip meter in em. Get the whole cluster with the diesel tach, it's a nice upgrade. Steve

 

[Pino2234]

Thanks Steve, I'll try that.

I know 80-86 is my dash cluster but I was wondering if the speedo cable will plug in the back of any speedo gauge from 80-96 and read correctly?

I also already have a tach in it but it doesn't seem accurate.

 

[Pino2234]

In my 1986 pickup (the one that the odometer was .3 miles off and 5 mph fast) I added a new speedo gear 1 tooth more and now the speedometer is right on but the odometer is slow. (more that .3) I am around 150miles on gps and like 140miles on my odometer.

What is correct odometer or speedometer or is GPS wrong?

 

[rjjp]

I know 80-86 is my dash cluster but I was wondering if the speedo cable will plug in the back of any speedo gauge from 80-96 and read correctly?

92 and up uses an electric speedo, other than that I'm no help.