Speedo adjustment device on T-Case

[CharlieC]

I knew what this was called at one point, but can’t remember now… This on my ‘74 NP203.



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

Thanks
Charlie

 

[ktmoutfront]

Speedometer Adapters.

Not sure if these guys still deal in them. Used to.

Speedometer Adapter - Install, Repair & Replacement

Speedometer adapter installation, & repairs to correct speed ratio inaccuracy. Mechanical, Electrical, drive cables & gears, odometer checks & recalibrations.

www.dicksspeedotach.com

I have always been a fan of doing it right, changing the gears internally.

 

[6872xtc]

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]

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]

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]

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]

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]

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.

 

[Kain]

found a used one on ebay that worked

 

[Joel Wilson]

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]

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]

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]

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...

 

[DrKessler]

* Speedometer Gear Calculator *

Speedometer Gear Calculator. Fill in the blanks to find your needed gear! Need assistance? Call Jim at 1-888-877-1008! Available Mon-Fri, 8am-5pm Central

transmissioncenter.net

haven't ordered it yet, but that's what i'm going to do for mine.

 

[CharlieC]

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]

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...