Speedometer Acting Strange - The 1947 - Present Chevrolet & GMC Truck Message Board Network

OldChevy1954 · 03-29-2023, 03:55 PM

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.

**HO455** · 03-29-2023, 04:55 PM

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

**Grumpy old man** · 03-29-2023, 09:54 PM

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

**'68OrangeSunshine** · 03-30-2023, 05:11 AM

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.

**special-K** · 03-30-2023, 07:05 AM

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

OldChevy1954 · 03-30-2023, 01:55 PM

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?

OldChevy1954 · 03-31-2023, 03:59 PM

Anybody know if TBONE1964 is still in business?

**'68OrangeSunshine** · 03-31-2023, 04:52 PM

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

03-29-2023, 03:55 PM	#1
OldChevy1954 Registered User Join Date: May 2016 Location: Salisbury, MD Posts: 58	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.

03-29-2023, 04:55 PM	#2
HO455 Post Whore Join Date: Feb 2016 Location: Portland Oregon Posts: 11,605	Re: Speedometer Acting Strange If TBone1964 doesn't chime in send him a PM. He's a member who rebuilds guages. __________________ Thanks to Bob and Jeanie and everyone else at Superior Performance for all their great help. RIP Bob Parks. 1967 Burban (the WMB),1988 S10 Blazer (the Stink10 II),1969 GTO (the Goat), 1970 Javelin, 1952 F2 Ford OHC six 4X4, 29 Model A, 72 Firebird (the DBP Bird). 85 Alfa Romeo If it breaks I didn't want it in the first place The WMB repair thread http://67-72chevytrucks.com/vboard/s...d.php?t=698377

03-29-2023, 09:54 PM	#3
Grumpy old man Senior Member Join Date: Dec 2013 Location: Gods country East,Tn Posts: 8,545	Re: Speedometer Acting Strange What trans do you have and does it have the Stewart Warner gear reduction box between the trans and cable ? __________________ 1967 Factory short bed - Old school '71 - 350 / 4bolt / 487 heads / Edelbrock C3BX Muncie M-22 4 speed / Hurst Comp plus Factory 12 bolt posi 3.73 / 255-70-15 Smoothed firewall / Factory cowl induction Power disc brakes / power steering / 3.5-5" drop

03-30-2023, 05:11 AM	#4
'68OrangeSunshine Senior Member Join Date: Feb 2002 Location: Tucson, AZ USA Posts: 7,622	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. __________________ Every 25 years I like to rebuild that 292, whether it needs it or not.

03-30-2023, 07:05 AM	#5
special-K Special Order Join Date: Nov 2004 Location: Mt Airy, MD Posts: 85,851	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 __________________ "BUILDING A BETTER WAY TO SERVE THE USA"......67/72......"The New Breed" GMC '67 C1500 Wideside Super Custom SWB: 327/M22/3.42 posi.........."The '67" (project) GMC '72 K2500 Wideside Sierra Custom Camper: 350/TH350/4.10 Power-Lok..."The '72" (rolling) Tim "Don't call me a redneck. I'm a rough cut country gentleman" R.I.P. ~ East Side Low Life ~ El Jay ~ 72BLUZ ~ Fasteddie69 ~ Ron586 ~ 67ChevyRedneck ~ Grumpy Old Man ~

03-30-2023, 01:55 PM	#6
OldChevy1954 Registered User Join Date: May 2016 Location: Salisbury, MD Posts: 58	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?

03-31-2023, 03:59 PM	#7
OldChevy1954 Registered User Join Date: May 2016 Location: Salisbury, MD Posts: 58	Re: Speedometer Acting Strange Anybody know if TBONE1964 is still in business?

03-31-2023, 04:52 PM	#8
'68OrangeSunshine Senior Member Join Date: Feb 2002 Location: Tucson, AZ USA Posts: 7,622	Re: Speedometer Acting Strange I have a GPS in my truck, so I have an accurate groundspeed to gauge the speedometer against. __________________ Every 25 years I like to rebuild that 292, whether it needs it or not.



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