Speedo Problem - Inconsistent - The 1947 - Present Chevrolet & GMC Truck Message Board Network

69GMCLonghorn · 07-05-2009, 07:58 PM

Ok, long story which I'll try to shorten up, whilst leaving in all the necessary details:

The old speedo was reading slow by 4 MPH, not by a percentage. Regardless of whether you're doing 30 or 60 MPH, it's off by 4 MPH. I was told the reason for this is the speedo was not properly "zero'd" when it was manufactured. The odometer was dead on accurate.

It was about 3x as much as buying a new one to get mine rebuilt. So I bought a new one from LMC. Now here is what's happening: The speedo will randomly read accurately (within 1 MPH per my GPS). BUT! At its own will, it will read slow at 40 MPH by as much as 5 MPH, and in the same run, will read fast by about the same at 70 MPH.

In English, if the speedo reads 40 MPH, I'm doing 45. If it reads about 76 MPH, I'm doing about 70 MPH... But not all the time! I called LMC and to their credit, they had no problem sending me a new one, which I installed. The same exact results are happening with this one.

Now, you are probably thinking what I am thinking: replace the cable. I'm going to do that, but why wouldn't this have been an issue with the old speedo???

Am I just getting crap units from LMC? Or do you think it's as simple as a cable problem, even though it wasn't an issue with the original speedo?

Help a brother out!!

gcburdic · 07-05-2009, 08:10 PM

It's probably not your speedo cable.....it maybe your driven gear??

If you have a TH350 tranny...try this thread....http://67-72chevytrucks.com/vboard/s...d.php?t=345091....
If you have a Th400...the speedo housing is a little different, but the change out is about the same....let me know, I have some pics of when I changed my TH400 speedo housing and driven gear out recently!!!

Hope this helps

68C10withA396 · 07-05-2009, 11:40 PM

Quote:

Originally Posted by gcburdic

It's probably not your speedo cable.....it maybe your driven gear??

If you have a TH350 tranny...try this thread....http://67-72chevytrucks.com/vboard/s...d.php?t=345091....
If you have a Th400...the speedo housing is a little different, but the change out is about the same....let me know, I have some pics of when I changed my TH400 speedo housing and driven gear out recently!!!

Hope this helps

If you still remember and have time, can you put a parts list together on what you had to buy to do the replacement on your 400?

69GMCLonghorn · 07-05-2009, 10:24 PM

Actually, I have neither. It's an NP435 manual trans. I'll still have a look at the gear and see and see if it looks worn, etc. Thanks!

gcburdic · 07-06-2009, 07:40 AM

No problem....I purchased a speedo housing....on a TH400 there are 2 different speedo housings pending the driven gear (I believe one is for gears 34-39 and another for 40-45)....the o-ring gasket that goes around the housing, and a driven gear (which I calculated by using the above thread)....I believe it cost me about $40 (I have a local tranny parts warehouse close to work)....hope this helps

07-05-2009, 07:58 PM	#1
69GMCLonghorn Registered User Join Date: Dec 2006 Location: Porter Ranch, CA Posts: 978	Speedo Problem - Inconsistent Ok, long story which I'll try to shorten up, whilst leaving in all the necessary details: The old speedo was reading slow by 4 MPH, not by a percentage. Regardless of whether you're doing 30 or 60 MPH, it's off by 4 MPH. I was told the reason for this is the speedo was not properly "zero'd" when it was manufactured. The odometer was dead on accurate. It was about 3x as much as buying a new one to get mine rebuilt. So I bought a new one from LMC. Now here is what's happening: The speedo will randomly read accurately (within 1 MPH per my GPS). BUT! At its own will, it will read slow at 40 MPH by as much as 5 MPH, and in the same run, will read fast by about the same at 70 MPH. In English, if the speedo reads 40 MPH, I'm doing 45. If it reads about 76 MPH, I'm doing about 70 MPH... But not all the time! I called LMC and to their credit, they had no problem sending me a new one, which I installed. The same exact results are happening with this one. Now, you are probably thinking what I am thinking: replace the cable. I'm going to do that, but why wouldn't this have been an issue with the old speedo??? Am I just getting crap units from LMC? Or do you think it's as simple as a cable problem, even though it wasn't an issue with the original speedo? Help a brother out!!

07-05-2009, 08:10 PM	#2
gcburdic Restoration Virgin! Join Date: Jan 2008 Location: Huntersville, NC Posts: 8,298	Re: Speedo Problem - Inconsistent It's probably not your speedo cable.....it maybe your driven gear?? If you have a TH350 tranny...try this thread....http://67-72chevytrucks.com/vboard/s...d.php?t=345091.... If you have a Th400...the speedo housing is a little different, but the change out is about the same....let me know, I have some pics of when I changed my TH400 speedo housing and driven gear out recently!!! Hope this helps __________________ Geoff with a G 1968 Chevy C10 SWB The Mistress 1972 Custom Deluxe C20 Suburban A Suburban Named Isabella 1966 Ford Mustang Mama's High School Graduation Present Last edited by gcburdic; 07-05-2009 at 08:12 PM.

07-05-2009, 10:24 PM	#4
69GMCLonghorn Registered User Join Date: Dec 2006 Location: Porter Ranch, CA Posts: 978	Re: Speedo Problem - Inconsistent Actually, I have neither. It's an NP435 manual trans. I'll still have a look at the gear and see and see if it looks worn, etc. Thanks!

07-06-2009, 07:40 AM	#5
gcburdic Restoration Virgin! Join Date: Jan 2008 Location: Huntersville, NC Posts: 8,298	Re: Speedo Problem - Inconsistent No problem....I purchased a speedo housing....on a TH400 there are 2 different speedo housings pending the driven gear (I believe one is for gears 34-39 and another for 40-45)....the o-ring gasket that goes around the housing, and a driven gear (which I calculated by using the above thread)....I believe it cost me about $40 (I have a local tranny parts warehouse close to work)....hope this helps __________________ Geoff with a G 1968 Chevy C10 SWB The Mistress 1972 Custom Deluxe C20 Suburban A Suburban Named Isabella 1966 Ford Mustang Mama's High School Graduation Present

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