speedo ratio adapter parts.

[Bigstevex4]

I swear i searched this becaue just read it but? Took adapter out hooked speedo up direct it works but is about 30% slow. The square drive on adapter is shot. parts or since larger tires best place new one thanks.

[rich weyand]

I assume you are talking about a K. I need details of the truck: year, model, transfer case, etc.

Give me an exact number for how slow, and I will look up the ACDelco ratio adapter part number. Go out on the interstate. Note your odometer reading (including tenths) as you pass a mile marker. Drive ten miles. Note your odometer reading (including tenths) as you pass that mile marker. Subtract the two. You should have something like 7.1 miles indicated. That let's me get the right adapter number. Then you just look up the GM part number on the net and buy it.

The keys are here. You need the HR124 and the HM146. No internet ordering, you call them on the phone and they ship them out same day or next day depending on whether the UPS truck has been and gone already today. They are about $10 total.
http://www.texasindustrialelectric.c...on_Adapter.asp

[Keith Seymore]

...

http://www.partshp.com/speedometer_gears.htm

K

[Bigstevex4]

Thanks it a c10 with a 465 i keep forgetting to check it furter will do tonite but 2 miles on interstate home is registers 1.45 miles but will drive it farther to get amore acurate read or between 1.4 1.5 need go 10 miles thanks will post later.

[Jeepwm69]

Let me know which one you end up getting. I have an 83 with SM465 that has the same problem. Going out and seeing how far off the speedo is currently has been on my list of things to do. I eliminated the box and hook my speedo up directly too, and it reads slow. I THINK I'm at about 55 when the speedo reads 45.

[hatzie]

I posted an Excel 97 spreadsheet I put together several years ago to calculate the Speedo gears and engine RPM at various highway speeds. I was doing a "What-If" on transmissions, tires, and axle gear-sets and wanted to get transmission/transfer-case speedo gears as close to correct as possible. My 76 reads 1-2mph higher than actual speed. If my speedo reads 35 the local police dept's "Your speed is this" sign will read 33 or 34.

For Speedo gears you only need to edit the Tire Size & Axle ratio. The speedo gear table will automagically calculate the ideal Driven and Drive gears.
You then get to find which ones you can actually buy. You can edit the ratio adapter section to get you closer... If the gears you can actually buy are not close enough.

For the RPM tables you'll need to edit the transmission and Transfer case gear ratios as well. There are tables of ratios found in common trannys and transfer cases in the RH pane.

The Free Apache Open Office and Libre Office 4.x and newer Suites "Calc" Spreadsheet applet will open and edit spreadsheets in Excel 97(XLS) & 2007 and newer (XLSX) formats. I lean toward Libre Office because the dev cycle for bugfixes and new features is shorter. Google Docs should open it on the web as well.
There are no Macros or VB Scripts. It's just a spreadsheet with some fairly complex formulas and color formatting.
I'll post a ZIP archive of it again here.

[craig113]

Bump, watching this thread also, will be adjusting mine in the next month as soon as the truck is together.

[86c20]

ok i am glad i found this. my wrecker has sm 465 and i have no speedo.. as in the cable is not hooked to anything. now here is where the problem lies. the cable is female end. so is the trans. no clue on what year anything is as its an 78-80 frame 81 cab trans is god only knows. was told i need a box between the tool but the one i found was external thread i think. been a few years dont remember all of what it was exactly. can some one point me in the right direction? plus what gear i would need for the 4.10? thanks

[rich weyand]

On the K trucks, the speedometer accuracy was achieved with a limited selection of gears for the transfer case (NP203 or NP205), and a ratio adapter to correct to 1000 revs per mile. On the C trucks, there were a lot more speedometer gear choices, at least for the automatics, and the adjustment was made in the gears. Not sure about the sticks.

86C20: I thought you had 3.42 gears in the hillbilly? It would be a lot easier to figure out what you needed if we had a baseline -- some reading like it's 12.5% slow -- but with no connection at all it's difficult. 4.10 gears with what tire diameter? Speedometer driven gears ran all the way from 16 to 25 teeth in one-tooth increments depending on ratio and tires.

[rich weyand]

Originally Posted by 86c20

ok i am glad i found this. my wrecker has sm 465 and i have no speedo.. as in the cable is not hooked to anything. now here is where the problem lies. the cable is female end. so is the trans. no clue on what year anything is as its an 78-80 frame 81 cab trans is god only knows. was told i need a box between the tool but the one i found was external thread i think. been a few years dont remember all of what it was exactly. can some one point me in the right direction? plus what gear i would need for the 4.10? thanks

OK, I just re-read this and see you're talking about the ramper. Do you have what you need for the speedo hookup on another truck with a 465? Like hillbilly? Because if you can put that on the ramper, and get an accurate read on how far off it is on the interstate, and then tell me what speedo driven gear (count the teeth) is in it, we can nail down which replacement one you need.

[86c20]

the main problem with it is i cant hook up the cable to the trans on the ramp at all. two female ends and the hillbill is a male female. plus the ramp is not on the road at this time( waiting for a new cab next winter body work paint is happing now) just seems like something is missing and was told that too but like i said its been some time for the last time i looked at it. want to get what i need so i can get it to work and see if it is off or what ever. the truck is so miss matched.

[Bigstevex4]

Mine ran up 7.1 miles in 10 mile markers so i need one thats 29% overdrive Seen one for sale thats 28% close enough.

[Jeepwm69]

Originally Posted by rich weyand

I assume you are talking about a K. I need details of the truck: year, model, transfer case, etc.

Give me an exact number for how slow, and I will look up the ACDelco ratio adapter part number. Go out on the interstate. Note your odometer reading (including tenths) as you pass a mile marker. Drive ten miles. Note your odometer reading (including tenths) as you pass that mile marker. Subtract the two. You should have something like 7.1 miles indicated. That let's me get the right adapter number. Then you just look up the GM part number on the net and buy it.

The keys are here. You need the HR124 and the HM146. No internet ordering, you call them on the phone and they ship them out same day or next day depending on whether the UPS truck has been and gone already today. They are about $10 total.
http://www.texasindustrialelectric.c...on_Adapter.asp

So Rich, are you saying that this place has the adapter box to hook the cable into an SM465? I'm not sure if I understand since you listed two part numbers there.

I know the link Keith posted sells adapters, and will even custom make one depending on reduction %, but wasn't sure if there was a less expensive option.

[rich weyand]

Originally Posted by Bigstevex4

Mine ran up 7.1 miles in 10 mile markers so i need one thats 29% overdrive Seen one for sale thats 28% close enough.

Uh, no. To get .71 up to 1, you need to multiply by 1.41, so you need a 1:1.41 step up.

Think of it this way. If I take 25% off of 1, I get 75%. But to get .75 up to 1, I need to add 33%.

[rich weyand]

Originally Posted by Jeepwm69

So Rich, are you saying that this place has the adapter box to hook the cable into an SM465? I'm not sure if I understand since you listed two part numbers there.

I know the link Keith posted sells adapters, and will even custom make one depending on reduction %, but wasn't sure if there was a less expensive option.

I was talking about K trucks with NP203 or NP205 transfer cases when I mentioned those adapters/keys. I think the Texas outfit can fix you up with whatever you need, though. It looks like they have pretty much everything. And they have a guy there that knows what he is talking about. As soon as you tell the gal qho answers the call that you want to talk ratio adapters, she'll transfer to him.

[Jeepwm69]

Thanks Rich. Gotta nail down the difference in current reading vs actual and then I'll give them a call.

[Bigstevex4]

Hey thanks i knew should paid more attentiaon in school. So they have a plus 38% or 44% so SWAG it would be roughly 1.5 mph fast or slow at 75mph?

[rich weyand]

Originally Posted by bigstevex4

hey thanks i knew should paid more attentiaon in school. So they have a plus 38% or 44% so swag it would be roughly 1.5 mph fast or slow at 75mph?

1.38 * .71 * 75 = 73.5

1.44 * .71 * 75 = 76.7

[RIDE-RED 350r]

OK... I have been searching this subject and I beg pardon for resurrecting a 3 month old thread. I need to correct the speedo on my '78 K-10.

I have given myself a migraine trying to figure the math on this.

Here is what I have:
-'78 K-10
-4-spd manual trans
-NP205 T-case
-3.73 axle ratio
-35" BFG tires

Here is what I know:
-my T-case limits my speedo drive and driven gear sizes. Ratio adapter best solution?
-When doing 60mph according to GPS speedo in my phone, speedo in truck reads appx 47mph.
-Speedo error lessens at lower speed. To go 30mph actual, speedo reads appx 23mph.

I would be thrilled if someone could post up the math formula I can use to determine the percentage I need to speed up my speedometer.

Also, being that my speedo cable connects T-case, will a np205 specific ratio adapter be needed??

Thanks!

[rich weyand]

1978 K-10 came with a ratio adapter in place from the factory. The one additional piece of information I need is the ratio stamped on the case of the ratio adapter that is already on there.

[RIDE-RED 350r]

Thanks Rich. I'll crawl under the truck tomorrow when I get home from work and post up the numbers

[RIDE-RED 350r]

Well Rich. I just crawled under the truck and guess what... No adapter in place whatsoever. My speedo cable plugs directly into the T-case.

I was going to pull the driven gear and see what I have, but I do not have a 1-1/16" socket as it appears the size of the driven gear holder is. I have no interest in damaging the holder trying to remove it with the wrong tool.

I will borrow a socket from work and pull it tomorrow to see exactly what I have.

From what I have been able to learn, and according to this page there are only 5 different size 205 pencil gears available, ranging from 18 to 22 teeth. The pencil gear is the one I was attempting to remove, right?

They also say I can only have 1 of 2 possible output shaft gears. An early metal type with 5 teeth, and a red later one with 6 teeth.

Here is a link: http://offroaddesign.com/NP205Tech.htm

So, since we probably cannot calculate what ratio adapter I need without knowing what drive output shaft and pencil gears I have, to be continued.....

[rich weyand]

All of what you say is true EXCEPT you don't need to know what is in the transfer case. Those parts are a bear to find!

I just wanted to know what adapter was on there already so you could get one adapter that would be that adapter corrected by 1.3 (30/23 or 60/47). With no adapter on there, all I needed was the actual and indicated speeds.

So you need a step-up adapter. 30/23 is 1.304, 60/47 is 1.276. Closest fit is 1.300, for which there are three: GM #3950350, GM #1565719, GM #368026. Not sure why there would be three, though my parts book covers 6 model years.

All that said, I can't find any of them on the net.

Closest I can find is "speed up 28%" aftermarket ratio adapter here:
http://www.transmissioncenter.net/sp...n_______va.htm

Anybody else know where to get a 1.300 output adapter?

[68Timber]

And the place Rich linked will make a custom ratio adapter for $79, I'm going to need one for my 85 to compensate for the 33" tires.

[RIDE-RED 350r]

Thank you so much Rich! That is great!

After learning that all of the 4x4's (at least those with the 205) came from the factory with a ratio adapter, you can imagine my surprise and chuckling when I found no such adapter in place on my truck.

If a 28% adapter gets me close, that will be more than sufficient.

So, 1.300 speed up is what, 30%? Sorry for the dumb math question. History was my strong suit in school. LOL!

Side note: It's all starting to make sense now. Previous owner bought it from Texas. Tried 4x4 for the first time and figured out pretty quickly that it had mismatched gear ratios front to back. Front was 3.73 and rears were 3.08. He got 3.73's back in. But that would explain the removal of the adapter. If someone is cheaping out and wants to change gear ratio without doing both axles, they would naturally do the rear and leave the front alone. But then it has 35" tires... HAHA! I give up.