speedo ratio adapter parts.

[RIDE-RED 350r]

Well that certainly is interesting.

And i have been playing with that spreadsheet. Still working on figuring things out.

That must have been a ton of work!

[hatzie]

Harder to find the formulas for the RPM, speedo gears, and tire sizes than to build them into a spreadsheet.
I'd say maybe 6-8 hours to get it tuned so it worked properly and color formatted so it was readable...

[rich weyand]

The GM ratios are specified as output per turn of input at the xfer case, so 1.300 means 1.3 turns to the speedo for 1 turn of what you called the pencil gear at the transfer case.

23*1.28 (28% faster) means you will read 29.5 at 30.

[RIDE-RED 350r]

Originally Posted by rich weyand

The GM ratios are specified as output per turn of input at the xfer case, so 1.300 means 1.3 turns to the speedo for 1 turn of what you called the pencil gear at the transfer case.

23*1.28 (28% faster) means you will read 29.5 at 30.

That will be outstanding!

Thank you so much for the help.

[RIDE-RED 350r]

Guess what Rich!!

I was sitting here thinking. And the thought crossed my mind that I have a ratio adapter sitting in one of the many boxes of parts my father in law sent me home with.

So, I took a walk out to the shed and had a quick look...

Guess what I found!

An SW adapter marked 777f 1.3000BH. I think it's the exact adapter I need! LOL!

I'm going to toss it in and see what I get!

[rich weyand]

Cool. Free is good.

[RIDE-RED 350r]

WAHOO!!!

Mission successful!

I put the ratio adapter in, fired up the GPS speed app on my phone, and took a spin. At 30mph actual, my speedo reads right about 31mph. At 60mph actual, my speedo reads 62-62.5mph.

I couldn't be happier, and I don't think I can expect much closer accuracy than this.

What are the odds that a box of miscellaneous parts from my father in law would contain the exact adapter I need??!! Especially with my tire/gear ratio combo..

Thanks again for you help with the math Rich. Without your input on what ratio I needed, I wouldn't have known what to look for.

[rich weyand]

No problem. Glad you got it sorted.

And yeah, that's close. Federal requirement is -0/+3, so that is as accurate as the mfr needs to make it.

Nice to find a $150 part laying around that is a perfect match.

[Bigstevex4]

Cool I lucked out out mine is as good as you can get 1% if the mileage counter is right. only hick up is the 1st # seems funny when got 900 +It moved up level turned over 70,000 couple days later it was hanging low at 80,000. But close enough. Speedo works great!

[hatzie]

Originally Posted by Bigstevex4

Cool I lucked out out mine is as good as you can get 1% if the mileage counter is right. only hick up is the 1st # seems funny when got 900 +It moved up level turned over 70,000 couple days later it was hanging low at 80,000. But close enough. Speedo works great!

The odometer driven gear in the speedometer head is cracked or someone reassembled it incorrectly after repairing a cracked gear or rolling back the odometer.

[Jeepwm69]

Just to necro a thread, finally put a GPS on my truck.

Speedo reads 45 when I'm going 57. That means I need a 1.26666 adapter right?

Speedo reads 35 when I'm going 45, so looks like the 1.26666 or the 28% increase adapter either one would put me close.

[rich weyand]

Originally Posted by Jeepwm69

Just to necro a thread, finally put a GPS on my truck.

Speedo reads 45 when I'm going 57. That means I need a 1.26666 adapter right?

Speedo reads 35 when I'm going 45, so looks like the 1.26666 or the 28% increase adapter either one would put me close.

Depends on whether there is an adapter on there already. If it is a K truck, there is probably already an adapter there, in which case, multiply the existing adapter ratio by the correction factor to get the required new adapter ratio. You don't want two adapters on there. If it is a C truck, yes, a 1:1.26 or 1:1.28 or 1:1.3 speed-up adapter is what you need.

GM didn't have a 1:1.26 or 1:128 adapter as standard parts on these trucks. The GM part number they would have used is 1565179, 1:1.300. These original adapters are very strudy, and also hard to find and expensive, $150 or so.

However, I did find this:
http://www.ebay.com/itm/1965-66-67-6...0eb813&vxp=mtr

[Jeepwm69]

This is a 2wd 83 GMC with SM465. The original adapter went kaput back in the early 90's and so a local shadetreen guy just removed the adapter and hooked the cable straight to the tranmission. I found a used one that says it's 1.2666. Sent you a link via PM.

Thank you for your help.

[CKE142B]

Where can i get NP205 speedometer gears parts?
I had my truck at a speedometer shop years ago. Original owner ran 33 x 12.50s and i went to LT235/75R15 tires (28" dia.).
Recently, speedo stopped working. Driven teeth are mashed, drive teeth worn. Driven gear has H (18 teeth), Drive gear #99408 stamped on face, Ratio converter box has 1.2020 X stamped on it.
Not sure if original or speedometer shop changed anything?

1972 Chevy K10 350V8, 4BBL, Muncie 465, NP205, 3.07:1 diff ratio.

[rich weyand]

https://www.google.com/#q=np205+spee...gears&tbm=shop

[CKE142B]

Thanks for the link. Checked the online sources. Mostly, everyone has the plastic gears with splines. I am not sure what the internal diameter is or if the plastic could replace a metal gear. Changing the output shaft to a newer style may be the next phase.
My local Chevy dealer located another dealer who has a drive gear in stock. Now, to find the driven gear and shaft.
Another option is to find a used set in good condition. Found a website: www.mmerlinn.com who has used transmissions and transfer cases.

1972 Chevy K10 Shortbed 350V8, 4BBL, Muncie 465, NP205, 3.07:1 diff ratio.

[rich weyand]

Another good link:
http://quad4x4.com/catalog/category/..._Parts#cat_262

Interesting conversation off the net:
Quote: Those gears inside should be all metal if I recall.
Yeah they will be up until about 1974-75,after that they splined the shaft and used a plastic gear.The pre 74ish 205's will not have splines that the speedo gears rides on,just has a metal gear and thrust washer.The gear is sandwiched between the bearing and thrust washer.

Finding the driven gear is a PITA. Part number for "H" 18-tooth is GM #3866798. I have actually found one, I think. PMed.

[Benjamin]

This might not be exactly the same as the issue being discussed in this thread, but it seems like one of you guys may be able to shed some light. I just swapped a junkyard SM465/NP205 (1985?) into my 1976 crew cab K20 which originally had a TH350/NP203. The original speedometer cable reached the speedo plug on the "new" NP205, so I made the connection and hoped for the best. Well, no such luck. The speedometer and odometer are not working AT ALL. Any ideas?
Thanks.

[hatzie]

Download a copy of the parts and parts illustration books they're free and they're darn handy.

Looked the 73-78 NP205 transfer case in the parts illustration book. Looks like GM only used two possible drive gear part numbers (#65) in the NP205 for those years (plastic or steel).
The Driven (output) gear can be several. The illustration book page only lists a section number. You'll need to look it up in the parts book.
The NP203 illustrations are just preceding the NP205 illustrations in the parts books.
The parts books will tell you whether you can use the driven gear from the NP203 or not.
There are drive:driven gear tables in the front of the parts books that you can look up to find the part numbers for the proper drive and driven gear for your tire size and axle ratio. (assuming GM offered that combination for your year).
The below pages are as far as I'll dig right now. You should be able to find the stuff you need in the books.

[Benjamin]

Well, it looks like the drive gear inside the t-case is worn out or stripped. I took out the sleeve and the gear (1 & 5) on both my old 203 and the "new" 205, and felt the gear inside the t-case that would drive No. 1 on the picture below. The teeth in 205 gear kinda feel a little less-than the teeth in the 203, so I'm guessing they just aren't driving the speedo gear. I put it all back together, then stuck a twig into the end of the driven gear where it connects to the speedo cable. When I rotate the rear output with my hand, the 203 would visibly turn the twig but the 205 does not. Looks like I need to change this gear. Anybody know how to do that? Also, would the 203 gear be interchangeable with the 205 gear?
Thanks,
Ben

[hatzie]

You have to remove the yoke and the output housing off the tail of the transfer case then you use a pick to release the spring under the speedometer drive gear. Drive is on the output shaft of the transmission/transfer-case. Driven is on the end of the speedo cable.
It's held on the same way as the drive gear on the 2wd transmissions. It's very easy to find the 2wd transmission speedo drive gear R&R instructions online.

TH700R4 speedometer drive gear R&R

The parts book lists 6273992 as the only plastic speedometer drive gear on the NP205 and the only speedo drive gear on the 203. GM used this same gear on the TH350 among other transmissions.
The drive gear is item 105 in the output section blowup below.

[Benjamin]

As noted above, the plastic internal speedometer drive gear is interchangeable between an NP203 and NP205. Getting the plastic gear out of the original 203 was easy enough, but opening up the 205 was a little more challenging. I had to unbolt the driveshaft and do all the work on my back underneath a stock-height truck....

Got it all put back together, took it for a test drive, and I have a functioning speedometer again.

Thanks for all the help hatzie, I couldn't have done it without you.

Stripped gear pictured below.

[gmc 2500 4x4]

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