Factory NP205 speedometer setup

[Toro]

Does anyone know if every 4WD with NP205 came from the factory with a speedometer gear adapter? I am the 2nd owner of a '72 Blazer that the PO left pretty much stock beyond adding a cup holder console. 350/TH350 with NP205 and 3.07 rear. Date tag still on transfer case, and tag still on rear diff cover. When I bought it, it had a 1.3 gear adapter which increases displayed speed. The speedometer has always been way fast. I removed the adapter and screwed the speedometer cable directly to the transfer case. A quick comparison to GPS has it showing really close, but I plan on driving a known distance of miles, and then checking how many miles speedometer shows as traveled to be sure. So now, I'm trying to figure out if these adapters were there always, or only with a config change when adjustment was necessary.

[geezer#99]

Tire height effects your speedo.
Maybe the original owner changed tire size and needed to correct it.
What tires were stock and what do you have now?

[mrein3]

Originally Posted by Toro

Does anyone know if every 4WD with NP205 came from the factory with a speedometer gear adapter? I am the 2nd owner of a '72 Blazer that the PO left pretty much stock beyond adding a cup holder console. 350/TH350 with NP205 and 3.07 rear. Date tag still on transfer case, and tag still on rear diff cover. When I bought it, it had a 1.3 gear adapter which increases displayed speed. The speedometer has always been way fast. I removed the adapter and screwed the speedometer cable directly to the transfer case. A quick comparison to GPS has it showing really close, but I plan on driving a known distance of miles, and then checking how many miles speedometer shows as traveled to be sure. So now, I'm trying to figure out if these adapters were there always, or only with a config change when adjustment was necessary.

I've never seen it written anywhere so what I think is how it works is based on what information I've gathered by we'll call it on the job training.

If your vehicle came with any axel gear ratio other than 3.73, you got an adapter. My truck has 3.73s, I run factory height tires, and my speedo is dead nuts on.

I swapped a th350/NP205 out of a Blazer that had 3.07's. The guy left the adapter on the case. I verified the gear ratio in the donor Blazer by writing down the code on the axel and looking it up later. I took off the adapter, kept the factory height tires, and again, I'm dead nuts on compared to a GPS or my phone or both.

In summary, I think all NP205s give you the correct speed on your speedo if you have factory size tires, no adapter, and 3.73 gears.

[Toro]

Spid shows it came with H78-15. It now has 235/75/15, which I believe are equivalent size to the H78-15. As I mentioned, the tag is present and I verified the tube stamping to be 3.07, but that doesn't mean something wasn't changed. I need to change gear oil, and I'll check for sure when I do. Usually with the higher ratio a speed reducer gear is needed, my was increasing speed by 1.3. Who knows what PO did, but at this point I bet the original was replaced in the thinking that all of those speed gear boxes were the same. Then again who knows for sure.

[geezer#99]

I’ve seen a lot of those ratio adaptors that don’t have the ratio stamped on them.

[roger0080]

Attached are two pages from the parts manual for a 1972 T350/NP205.

You could have two different driven-gear/adaptor combinations for the 3.07 depending on which H78 tires you had (Highway or On/Off-Road).

In your picture showing the "1.3000J" stamped in the adaptor, I presume that the "J" refers to code "J", Adaptor 3950350 as shown in the parts table?

It would have been helpful if they had a column in the table listing the ratio for each adaptor, i.e. 1.3000

After seeing this, I have to go back and look at my 71 K20 that I swapped out my T350/NP205 w/4.10 from a '77 SWB CH465/NP205 gear ratio of ?. I put an adaptor on that and got it back to pretty close but the speedo is still off. I should be able to use the old T350/NP205 speedo gear w/o the adapter according to the parts manual table. It's winter here now so I'll have to look at that in the spring.

Summary: One needs the correct driven-gear -and- correct adaptor (if necessary) for your gear ratio. Sigh. See attached pic's.

Attachment 2238214

[Toro]

Originally Posted by roger0080

Attached are two pages from the parts manual for a T350/NP205.

You could have two different driven-gear/adaptor combinations for the 3.07 depending on which H78 tires you had (Highway or On/Off-Road).

In your picture showing the "1.3000J" stamped in the adaptor, I presume that the "J" refers to code "J", Adaptor 3950350 as shown in the parts table?

It would have been helpful if they had a column in the table listing the ratio for each adaptor, i.e. 1.3000

After seeing this, I have to go back and look at my 71 K20 that I swapped out my T350/NP205 w/4.10 from a '77 SWB CH465/NP205 gear ratio of ?. I put an adaptor on that and got it back to pretty close but the speedo is still off. I should be able to use the old T350/NP205 speedo gear w/o the adapter according to the parts manual table. It's winter here now so I'll have to look at that in the spring.

Summary: One needs the correct driven-gear -and- correct adaptor (if necessary) for your gear ratio. Sigh. See attached pic's.

Attachment 2238214

Thanks for this info. The chart list driven gear and sleeve assembly, is that inside the NP205? If so, I haven't counted teeth in there to see what I have. As you pointed out, both the driven gear and adapter changed based of gear ratio and tire size.

[jeffahart]

You can try flipping the gears in the adapter. I flipped mine and got lucky because now my speedo is dead on. Here is what the insides look like. Mines an S.S. White, but should be how your's looks on the inside too. If nothing else you can clean it up and get some new grease in there.

j

[Accelo]

Some adapters flip and others do not. The one at the top of the page can not be flipped.

[jeffahart]

Originally Posted by Accelo

Some adapters flip and others do not. The one at the top of the page can not be flipped.

Why not?

[Accelo]

One is male and female. The other is two male fittings.

[jeffahart]

Originally Posted by Accelo

One is male and female. The other is two male fittings.

You can put your adapter inline between two cables. You can get a short cable, that works like an adapter. Male-Male Female-Male --- and so on. Then you can go on ebay and find an adapter with the ratio you want and use cables to adapt thread configuration. I don't think I'd spend too much time looking to re-gear the adapter. Probably easier to get the whole adapter. If you find a male-male in the correct ratio, just get a short cable and put between the transfer and adapter.

Good luck,

j

[mrein3]

I got curious and looked at my adapter. Then I got more confused.

Those charts call out lots of information but it seems like you need one more chart. Or I should say GM needs one more chart. Or like roger0080 points out, another column. Something is missing.

I dug out my adapter that I currently don't use. It has the same 666F number in the top stamping area but followed by 010 where Toro's has E4. That makes no sense looking at those charts. I thought the bottom stamping on my adapter was a date code or something until I zoomed in on the picture. It clearly has a decimal point making that number a 1.2020, likely a ratio where Toro's is a 1.3000.

Maybe all the drive gears are the same and only the driven gear matters. The adapter jaffahart has taken apart shows two similar gear driven parts, an input and an output for lack of better terms. Maybe the "sleeve" in the GM chart is the part that is your output and that double gear that appears to go between them is the "Driven Gear" with the "T" number which is likely tooth count. Then if "Sleeve Assy" is output, maybe the T number goes with it? I don't know.

The more I think about this the less it makes sense so I am going to stop.

FWIW, the adapter I have is a "male/female" style. The mail end threads on to the t-case like your speedo cable then the speedo cable threads on the female end.

One more piece of the puzzle. I doubt the NP-205 has variation in it's internal gears. In other words there probably isn't a variable introduced into this problem from the transfer case. Your variables are tire size and rear end gear ratio.

[Accelo]

In years past I could go into the local parts dealer, that stocked Delco parts, and purchase this box. They would set it up to any possible gear ratio with the parts they had in stock. Those days are gone of course.
This conversion box is used to correct speedometer errors as not all needed gear ratios were available using combinations of the two gears on the transmission shaft. Even without incorporation of the extra corrections box the speedometers are typically off when new. A 1 to 3 percent error seemed to be acceptable. Typically, GM went with the ratio that would be 1 to 3% slow for obvious reasons.

[roger0080]

I did some googling the the adaptor part numbers shown in the 72TL parts book previously posted.

In later year parts books they list the missing info: the ratio and the associated stamp code that's missing from the 72TL parts book.

Assuming no changes in the GM part numbers or tooth counts up through the 80's, this is what I have found for the ratios and stamp codes. See attached pic's.

I cannot find any details on 3925671 which is not specified much.

For the NP205, 3.07, H78-15B tires you need adaptor:
3950348 1.2020X (on/off road)
-or-
3950350 1.3000J (highway)
as marked in red in the first picture.

There is a seller on the parts board selling the 1.2020X http://67-72chevytrucks.com/vboard/s...d.php?t=840267