CK5 Network
Become a member of the largest online community for Chevy K5 Blazer, Suburban and C/K series truck owners!

Speedo adjustment device on T-Case

CharlieC

Registered Member
Premium Member
Joined
May 28, 2007
Posts
57
Reaction score
15
Location
Flower Mound, TX
I knew what this was called at one point, but can’t remember now… This on my ‘74 NP203.

6C8FACBD-68AF-48DB-98A9-DB619D53371D.jpeg

is anyone still selling the parts or is there something similar to adjust for tire/gear changes?

Thanks
Charlie
 

6872xtc

make mine modified
Premium Member
GMOTM Winner
Joined
Sep 16, 2008
Posts
7,785
Reaction score
8,035
Location
Palisade,Colorado
What are you needing?
There are probably lots of those laying around used, you just need to know what you need for ratio.
Those were very common back before electric speedometers came out. I saw them a lot on semis, we would have to open them up to change the grease in them every so often.
 

CharlieC

Registered Member
Premium Member
Joined
May 28, 2007
Posts
57
Reaction score
15
Location
Flower Mound, TX
Was busy with work today and couldn‘t call the company mentioned above.

Given I went through the trouble of cleaning it off, I figured it was worth a shot to see about getting the correct gear combo to address 4:10 gears and 35’s. I’ve seen a couple of charts online, but can’t make heads or tails of them as it relates to the markings on my unit.

Charlie
 

6872xtc

make mine modified
Premium Member
GMOTM Winner
Joined
Sep 16, 2008
Posts
7,785
Reaction score
8,035
Location
Palisade,Colorado
Your box is an overdrive unit. The ratio is 1.2020.
You will need to know which direction to go from what you have.
What is the speedometer reading with the box installed?
What is the actual speed of the truck?
The difference between those two readings will help determine what to do. If your speedometer is reading too fast, the box is adding too much.

As far as replacing the drive or driven gears in the transfer case to correct the speedometer, I have no idea where you can find the parts. I haven't searched for those parts for a 208. Maybe someone can help there, however you will need to know what gear set you have currently.
 

CharlieC

Registered Member
Premium Member
Joined
May 28, 2007
Posts
57
Reaction score
15
Location
Flower Mound, TX
With 3:something gears (I’m guessing 3:07 or 3:42, it’s been woefully undergeared it’s whole life) and the adapter, I’d run about 5-6 miles slow at indicated 60mph. I’m guessing with the 4:10’s it might run fast? I think you’re saying today, the adapter is slowing the Speedo down?

Thanks
Charlie
 

6872xtc

make mine modified
Premium Member
GMOTM Winner
Joined
Sep 16, 2008
Posts
7,785
Reaction score
8,035
Location
Palisade,Colorado
My memory says that the 1.20 is speeding it up. I would pull the gearbox off of the speedometer and try it. Check it against a GPS app on your phone.
You can also verify the ratio of the box once you pull it off. Just rotate the input by hand to see how far the output turns for one revolution of the input.
 

bradshawtech

1/2 ton status
Premium Member
Joined
Jul 28, 2007
Posts
1,195
Reaction score
236
Location
Monte Vista, Colorado
With 3:something gears (I’m guessing 3:07 or 3:42, it’s been woefully undergeared it’s whole life) and the adapter, I’d run about 5-6 miles slow at indicated 60mph. I’m guessing with the 4:10’s it might run fast? I think you’re saying today, the adapter is slowing the Speedo down?

Thanks
Charlie
Try running without the adapter and checking accuracy of the speedo.
 

Joel Wilson

1/2 ton status
Premium Member
Joined
Jun 7, 2017
Posts
308
Reaction score
154
Location
NorCal
With 3:something gears (I’m guessing 3:07 or 3:42, it’s been woefully undergeared it’s whole life) and the adapter, I’d run about 5-6 miles slow at indicated 60mph. I’m guessing with the 4:10’s it might run fast? I think you’re saying today, the adapter is slowing the Speedo down?

Thanks
Charlie
It's early for me but I don''t think gearing has anything to do with the speedometer being off. I thought it was going with larger tires vs. stock size that would cause the speedo to be off. With a larger tire you are going a longer distance for the same rotation of the axle shaft.
 

CharlieC

Registered Member
Premium Member
Joined
May 28, 2007
Posts
57
Reaction score
15
Location
Flower Mound, TX
It's early for me but I don''t think gearing has anything to do with the speedometer being off. I thought it was going with larger tires vs. stock size that would cause the speedo to be off. With a larger tire you are going a longer distance for the same rotation of the axle shaft.
Hmmm, I guess I need to drink some more… I had a ’96 Impala SS I did a gear swap up to 3:42’s and had to reprogram the PCM to slow the speedo down. I’m thinking the same logic applies here, but I’m not sure now trying to think it through. My assumption was Chevy put those adapters on trucks to account for different gear setups, not different tire setups as the stock tires back then were fairly uniform in size, but maybe I’m wrong…. We’ll find out for sure when I drive it.

Charlie
 

Blue85

Yooper
Premium Member
GMOTM Winner
Joined
Jul 26, 2000
Posts
12,017
Reaction score
3,446
Location
Keweenaw
There were only so many drive and driven speedo gears available for each transfer case. I believe that's where the factory ratio adapters came from. In general I would consider the adapter plan B and try to do it internally, if possible, but all these decades have only made some of those gears harder to find.
 

nitro_rat

Registered Member
Premium Member
Joined
Jul 30, 2005
Posts
66
Reaction score
8
Location
Lufkin, Texas
There's a guy on ebay that will send you the gears you need, just tell him the ratio and tire size you're running. Don't remember his name but I can always find him with a quick search...
 

CharlieC

Registered Member
Premium Member
Joined
May 28, 2007
Posts
57
Reaction score
15
Location
Flower Mound, TX
There's a guy on ebay that will send you the gears you need, just tell him the ratio and tire size you're running. Don't remember his name but I can always find him with a quick search...
Can you help a brutha out with a link? I see several adapter auctions, but don't see anything for just the gears.

Thanks!
Charlie
 

nitro_rat

Registered Member
Premium Member
Joined
Jul 30, 2005
Posts
66
Reaction score
8
Location
Lufkin, Texas
www.ebay.com

Type "NP203 Speedometer Gears" in the search bar. Click on any listing by LA Speedometer and you will see that they say "if you need assistance determining what gears you need send us a message." Send them a message and tell them what you need. They will message you back and generate a listing for what you need. Click "Buy it Now" and you'll get it in the mail in a few days...
 
Top Bottom