Speedometer Acting Strange
 

My 72 K20 has a something unusual happening all of a sudden. I replaced the speedo with an LMC unit a few years ago and it has worked fine. Accurate too since I put the right gear in the transfer case.

Now it's approximately 15 to 20 mph off at higher speeds. Sometimes its as high as 40 mph too fast.

What could be going on here?

Thanks.

Re: Speedometer Acting Strange
 

If TBone1964 doesn't chime in send him a PM. He's a member who rebuilds guages.

Re: Speedometer Acting Strange
 

What trans do you have and does it have the Stewart Warner gear reduction box between the trans and cable ?

Re: Speedometer Acting Strange
 

If you disconnected the speedo cable at both ends, you could pull the spiral flex rod from the Cab end and inspect it. If it's OK, then squirt some canned air to clear the tube. Before you re-insert the flex Rod, squirt puffs of Powdered Graphite into the tube. Then add more puffs of
the sticky, black powder on the sides of the Shaft as it goes back down the tube.
I had an original Speedometer jumping around above 60 before I lubed it with Graphite Powder. Now it bounces a lot less.
Beware of black smudge, however.

Re: Speedometer Acting Strange
 

If pulling the speedometer cable from the housing, cleaning, and lubing doesn't fix it, which is always a good thing to do on a 50+ year old truck, then it's a poorly made aftermarket speedometer. I have never seen such an issue with an OEM speedometer. Usually, the way a speedometer tells you the cable needs cleaned and lubed is by the needle movement being jumpy. If it's really bad it will be very unresponsive and inaccurate

Re: Speedometer Acting Strange
 

It's an LMC speedometer only 3 or 4 years old.
I have replaced the cable a couple times since I owned the truck
It does not have a speed reduction unit on it at this time. I was able to get the proper GM plastic gear for inside the transfer case to get me real accurate 3 years ago.

The speedo cable goes to the side of the NP205 transfer case.

It was working real good until recently. I had someone drive 60 mph next to me and my speedo was saying 75. It's more accurate at lower mph, then as I get up around 65 the difference is exaggerated. More speed = more crazy reading.

It does not jump around at all.

I'm thinking something inside the head unit itself?

Re: Speedometer Acting Strange
 

Anybody know if TBONE1964 is still in business?

Re: Speedometer Acting Strange
 

I have a GPS in my truck, so I have an accurate groundspeed to gauge the speedometer against.