Author Topic: Factory speedo gears?  (Read 2442 times)

Offline Scaryfastatx

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Factory speedo gears?
« on: November 23, 2023, 03:08:41 PM »
Question for you all

Did the factory use a fixed set of speedo gear combos on these trucks?

Is there a resource on what the factory used for certain equipment?

My speedo is showing about ~35mph when I'm doing ~22mph

I don't have a tach, so just trying to research what came in these to get an idea while I wait to get a tach to see what my engine speed is compared to my velocity

Thanks!

Offline zieg85

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Re: Factory speedo gears?
« Reply #1 on: November 23, 2023, 05:04:54 PM »
There are 3 things, the drive gear on the output shaft, driven gear in the housing that the speedometer cable attaches to and possible another adaptor that does fine adjustments if the combination isn't just perfect.  Attached is an example of what is found in the GM manuals
Carl 
1985 C20 Scottsdale 7.4L 4 speed 3.21
1986 C10 under construction
https://www.facebook.com/groups/248658382003506/

Online Mr Diesel

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Re: Factory speedo gears?
« Reply #2 on: November 23, 2023, 06:45:49 PM »
These speedometers are based on a magnet system that inevitably deteriorates with age, causing inaccuracy. Best way to get your speedometer accurate is buy several different gears for the tranny side and experiment with each, using your phone GPS to check each one for accuracy.
1976 C20 Crewcab, 6.2L/SM465
1982 K30 Crewcab , 427TD/TH400
1983 C30, 6.2L/TH400
1983 K30 Crewcab 454/700R4
1986 K10 350/400. 1989 K30 cab/chassis 454/SM465

Offline JohnnyPopper

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Re: Factory speedo gears?
« Reply #3 on: November 23, 2023, 07:28:56 PM »
That's great advice Mr. D.

I looked at the chart saying drive and driven gears. Drive is obviously on the output shaft and driven on the end of the speedo cable. Where does the 'adapter' come in? Sorry to sound so clueless... :-[
1957 Apache 3100 235 Inline 6, 3 on the tree
1973 C-20, 3+3 454 4BBL TH400  Water Injection
1978 K-10, 350 4BBL TH350 NP203 M.M. Part time Kit/Hubs
1980 C-10 under construction

Offline Scaryfastatx

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Re: Factory speedo gears?
« Reply #4 on: November 24, 2023, 03:51:15 PM »
There are 3 things, the drive gear on the output shaft, driven gear in the housing that the speedometer cable attaches to and possible another adaptor that does fine adjustments if the combination isn't just perfect.  Attached is an example of what is found in the GM manuals

Cool thanks, so my truck had factory cruise control, but I bypassed it and went direct, would that cause a problem? I figured it would be 1:1 thru that cruise control box


Offline Scaryfastatx

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Re: Factory speedo gears?
« Reply #5 on: November 24, 2023, 03:53:09 PM »
These speedometers are based on a magnet system that inevitably deteriorates with age, causing inaccuracy. Best way to get your speedometer accurate is buy several different gears for the tranny side and experiment with each, using your phone GPS to check each one for accuracy.

That magnet system is on the cluster?

Mine was hanging up and staying at like 15mph, I thought initially that was the problem, but after some wd40 it's falling to zero but still off

Offline zieg85

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Re: Factory speedo gears?
« Reply #6 on: November 25, 2023, 12:31:50 PM »
There are 3 things, the drive gear on the output shaft, driven gear in the housing that the speedometer cable attaches to and possible another adaptor that does fine adjustments if the combination isn't just perfect.  Attached is an example of what is found in the GM manuals

Cool thanks, so my truck had factory cruise control, but I bypassed it and went direct, would that cause a problem? I figured it would be 1:1 thru that cruise control box

Taking the cruise out of the equation doesn't change a thing.  If what Mr. Diesel is saying is true in your case your odometer still should be correct.  If it is the wrong gear combination for every mile you travel the odometer would show 1.6 miles traveled.  That should be easy enough to verify.
Carl 
1985 C20 Scottsdale 7.4L 4 speed 3.21
1986 C10 under construction
https://www.facebook.com/groups/248658382003506/

Offline zieg85

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Re: Factory speedo gears?
« Reply #7 on: November 25, 2023, 12:39:25 PM »
That's great advice Mr. D.

I looked at the chart saying drive and driven gears. Drive is obviously on the output shaft and driven on the end of the speedo cable. Where does the 'adapter' come in? Sorry to sound so clueless... :-[

The adaptor as seen in the last column attaches where the speedo cable attaches at the trans or transfer case.  This does the fine adjustments.  Attached is one version

Carl 
1985 C20 Scottsdale 7.4L 4 speed 3.21
1986 C10 under construction
https://www.facebook.com/groups/248658382003506/

Offline JohnnyPopper

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Re: Factory speedo gears?
« Reply #8 on: November 25, 2023, 04:09:53 PM »
Thx Zeig! I'm smarter today...
1957 Apache 3100 235 Inline 6, 3 on the tree
1973 C-20, 3+3 454 4BBL TH400  Water Injection
1978 K-10, 350 4BBL TH350 NP203 M.M. Part time Kit/Hubs
1980 C-10 under construction

Online Mr Diesel

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Re: Factory speedo gears?
« Reply #9 on: November 25, 2023, 09:52:19 PM »
Here are some pics showing the magnetic system that results in your speedo reading. Basically, the cable inserts into back of speedo and spins a magnet. The needle is attached to a mildly magnetic disc, which is attracted to the spinning magnet but spaced far enough apart not to touch. The faster the cable/magnet spins the more the magnetic disc/needle turn to show increase in speed. A spring is also constantly acting on the needle to return it to 0 mph when the cable and magnet are not turning. Very simple system but subject to weakening of the magnet over time.

I'm not familiar with the piece shown by Zieg. Had many square bodies but never have seen one of those attached to transmission. Seems like a nifty thing to have if you can fine-tune the cable speed with it.
« Last Edit: November 25, 2023, 09:55:25 PM by Mr Diesel »
1976 C20 Crewcab, 6.2L/SM465
1982 K30 Crewcab , 427TD/TH400
1983 C30, 6.2L/TH400
1983 K30 Crewcab 454/700R4
1986 K10 350/400. 1989 K30 cab/chassis 454/SM465

Offline JohnnyPopper

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Re: Factory speedo gears?
« Reply #10 on: November 25, 2023, 10:44:47 PM »
Yes! An invention credited to Nicolas Tesla.

One of many ideas that he never patented... 8)
1957 Apache 3100 235 Inline 6, 3 on the tree
1973 C-20, 3+3 454 4BBL TH400  Water Injection
1978 K-10, 350 4BBL TH350 NP203 M.M. Part time Kit/Hubs
1980 C-10 under construction

Offline Chevygold

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Re: Factory speedo gears?
« Reply #11 on: December 07, 2023, 02:43:16 PM »
I used one of the online calculators to determine which gears I needed when I swapped my broken TH350 for a 400, I measured my tyre radius from centre to the road, put in the diff ratio and went for the suggested gears and it came out spot on, my TH350 had an adaptor but I did away with it for the 400.
Graham

Offline Scaryfastatx

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Re: Factory speedo gears?
« Reply #12 on: December 08, 2023, 08:59:13 PM »
I'm still waiting on the cheapo tach I ordered to come in

But based on the factory specs for the Speedo gear usage and the specified tire size and rear Gear ratio, I'm pretty certain there truck has 4.11 gears out back

I'm gonna jack it up this weekend and spin the drive shaft to verify