Factory speedo gears?

[Scaryfastatx]

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!

[zieg85]

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

[Mr Diesel]

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.

[JohnnyPopper]

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...

[Scaryfastatx]

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

[Scaryfastatx]

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

[zieg85]

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.

[zieg85]

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

[JohnnyPopper]

Thx Zeig! I'm smarter today...

[Mr Diesel]

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.

[JohnnyPopper]

Yes! An invention credited to Nicolas Tesla.

One of many ideas that he never patented...

[Chevygold]

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

[Scaryfastatx]

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