Speedometer/NP203 woes -- I need some help

[rich weyand]

I'd like to tap the collective experience of the group a bit.

The speedometer on my '78 K-10 stopped working.  I have 350/TH350/NP203, and the NP203 is a part-time conversion.  The NP203 has a 90* drive adapter on it, then I added a 10% step-down box when I put the 31" tires on it.  It had 12-year-old 33s on it when I got it.  Before it became inop, the speedo was dead-on.

So I go in there to look at it, and I pull off everything.  The speedo take-off on my NP203 is on the passenger side, which I think is not normal, so God knows how they did that.

Instead of the square drive plastic female on the end of the NP203 driven gear, I have a tang adapter (like the tang end on the right side of the piece in the first picture below) and on the other end of that adapter is a weird knurled end that was apparently driven into the metal tube that the little plastic square female is supposed to be in.  It looks kind of like the piece in the second picture.




So the driven gear does not have the plastic square drive in it anymore, and the end of the driven gear instead has this deep hole in it.  I tried driving this knurled piece back into it, and I had speedometer function for about 2 miles before it failed again.

Looking for NP203 driven gears, I haven't found a source.  Also, apparently there are 4 different tooth-count gears depending on rear-end ratio, tires and the like.

Back to the drawing board.  The truck RPO sheet says it has RPO HE4 3.40 axle ratios, but I've learned never to underestimate the POs on this truck.  Now that I have a tach, I can figure out the rear-end ratio from first principles.  At 60 mph, I am doing 2500 rpm more or less.  If I assume a 1:1 rear end ratio for the moment, 2500 rpm*60min/hour*((31 inch tires*pi)/(12 in/ft))/5280 ft/mile is 230 mph with 1:1 rear-end ratio.  Since I was going 60 mph, dividing out, it appears I have 3.78:1 axles on the truck.

Well, that's a surprise.  So my questions are:

- Does that calculation look right?

- What driven gear tooth count (14, 15, 16, or 19) should I use?

- Would it just be simpler to hook up to the TH350 speedometer output and "divide by 2" on the (very few) occasions I might be in LO range?

- What gear(s) would I need for the TH350 to do that, with 3.78:1 and 31" tires?  I could also use the gears for 3.78:1 and standard tires, and hook up my 10% gear reducer.  It's a step-down box now, but I think I can swap the gears around internally for step-up.

In the meantime, I have the tach, and the Garmin GPS also reads out in mph, so not an emergency, but I would like to get this sorted properly.

Thanks for any help!

[Dan75k20]

That's weird I have never seen a spedo hook up on the right hand side

[rich weyand]

That's weird I have never seen a spedo hook up on the right hand side

Yeah, beats me.  Maybe they put the tail cone back on rotated 180 when they modded the NP203 for some reason.

Anyway, climbing around under the ruck and surfing the net and the parts book, I've learned some stuff.

- The TH350 on the K's does not have the tail cone where the speedometer outlet is.  The NP203 bolts up right behind the governor.

- In 1978, GM apparently used the same drive and driven gear on every K truck, and then used a 90* adapter with a built-in ratio correcter for each combination of tires and axle ratio.  They had ten different ratio adapter/90* drive units used in production.  Each has the ratio stamped on the case, along with the GM part number, which certainly makes things easy for changing tire sizes.

- For 3.78:1 and 33" tires, use 1:1.300.  That's how my truck was set up when I got it.  You can work out the others from that.  I added a 10% reducer for 31" tires, but now that I am redoing everything under there, I am moving to the 1:1.200 adapter for the 31" tires on 3.78:1 axle ratio.

So I have coming: a new driven gear, because the one that is on there is buggered; a new sleeve, the widget that screws into the NP203 and has the speedometer cable threads on it, and includes the O-ring seals that keep all of the above from leaking; a new ratio/90* adapter.  This will put all of that in stock configuration and get rid of the jury-rigging that's been done.

[Dan75k20]

Well where is your vent tube at on the back of the 203

Sent from my Nexus 5 using Tapatalk

[rich weyand]

Well where is your vent tube at on the back of the 203

Sticking out the top.  Here's what mine looks like.  Definitely a 203.  Note fill plug location and lack of idler shaft cover.  Note also speedometer output location.  That's exactly how mine looks.