Speedometer adjustments

[Kenneth W Larew]

I know some systems allow adjustment via gears for larger tires etc...but with the speedometer cable going into my 4×4 case I can't see a way of doing this. Anyone know a way of calibrating speed on an 85 k10?

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[hatzie]

THere's a drive gear on the transfer case output shaft and a driven gear in driven gear carrier. The NP208C uses the same gears and driven gear carrier as the TH700R4.
If you need to swap the drive gear you'll have to remove the tailshaft housing from the transfer case. You can do this with the TC installed but make sure there's not a bunch of dirt on top to fall on the output shaft.

The parts manuals have pages of tables to lookup the correct combination of gears based on your model-year, tire-size, transmission or transfer-case, and axle ratio... Download the 85-91 parts books from my signature. The tables are in the front of the parts book.

 

[HotRodPC]

The ones that are adjustable are electronic with VSS and a DRAC chip. Your's is NOT electronic so you can get an adjustable adapter that change by moving, removing or adding pins and they pretty much suck or you can change your driven and drive gears in the Transfer Case. If you're lucky just the driven gear needs changed and usually is the case for just a tire change but if you make a drastic gear change that usually requires drive gear change too.

 

[Kenneth W Larew]

Thanks guys!

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[Craig 85]

GM used these ratio adapters too, my '79 K15 had one. There are companies that still offer them. Basically you drive 10 miles and come up with a percentage you're off. They make the adapter that will correct it.
http://www.gaugeguys.com/ratio-adapter-1.html

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[donnieray]

GM used these ratio adapters too, my '79 K15 had one. There are companies that still offer them. Basically you drive 10 miles and come up with a percentage you're off. They make the adapter that will correct it.
http://www.gaugeguys.com/ratio-adapter-1.html

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Thanks Craig, I'd rather buy one of these and leave the stock set up in my tc.

 

[Kenneth W Larew]

GM used these ratio adapters too, my '79 K15 had one. There are companies that still offer them. Basically you drive 10 miles and come up with a percentage you're off. They make the adapter that will correct it.
http://www.gaugeguys.com/ratio-adapter-1.html

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Over $100 shipped though. Makes deciding tough. I think the gears are less than $20...but a lot more work.

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[Craig 85]

They haven't gone up that much over the years. I think I was paying about $75 for them back in the early 90's. Saves a lot of time and aggravation.

 

[donnieray]

They haven't gone up that much over the years. I think I was paying about $75 for them back in the early 90's. Saves a lot of time and aggravation.

And the only thing I've changed on my 84 K is the tire size, so if and when I put the stock Ralley wheels back on I'll have no problems.

 

[Kenneth W Larew]

And the only thing I've changed on my 84 K is the tire size, so if and when I put the stock Ralley wheels back on I'll have no problems.

That's what I'm thinking....bite the bullet, keep everything stock and it'll be an easy switch back down thy he road.

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[78C10BigTen]

GM used these ratio adapters too, my '79 K15 had one. There are companies that still offer them. Basically you drive 10 miles and come up with a percentage you're off. They make the adapter that will correct it.
http://www.gaugeguys.com/ratio-adapter-1.html

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Ive seen 4 or 5 of these recently in the local junk yard

 

[hatzie]

I never liked the ratio adapter boxes. One more part to fail.

The driven gear isn't a big deal to change. There's a removeable carrier in the side of the transfer case that the speedo gear screws onto.

The drive gear isn't that much more involved. Pull the driveshaft out and remove the tail housing. The gear slides off the end of the output shaft. Gives you an excuse to change the seals on the tailshaft housing too.

Keep the old gear(s) in a ziploc freezer baggie in the glovebox.

This is a TH700R4 on the bench... No more difficult under the rig. Just make sure it's clean on top before you pull off the aluminum extension housing.

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[HotRodPC]

Over $100 shipped though. Makes deciding tough. I think the gears are less than $20...but a lot more work.

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It may not be alot of work. Are you positive that you'd also need to change the gear on the output shaft? If you don't need that one, it's a 10 minute job to change the side gear.

 

[HotRodPC]

I never liked the ratio adapter boxes. One more part to fail.

The driven gear isn't a big deal to change. There's a removeable carrier in the side of the transfer case that the speedo gear screws onto.

The drive gear isn't that much more involved. Pull the driveshaft out and remove the tail housing. The gear slides off the end of the output shaft. Gives you an excuse to change the seals on the tailshaft housing too.

Keep the old gear(s) in a ziploc freezer baggie in the glovebox.

This is a TH700R4 on the bench... No more difficult under the rig. Just make sure it's clean on top before you pull off the aluminum extension housing.

xc_hide_links_from_guests_guests_error_hide_media

And they do fail. My dad bought one of those when he did a gear chamge. Paid $200 to have his speedo calibrated with a 30 day warranty. In 30 days, they replaced it 2 more times. At 40 days it broke again, SORRY, only 30 day warranty. So he's not had a speedo since.

 

[hatzie]

And they do fail. My dad bought one of those when he did a gear chamge. Paid $200 to have his speedo calibrated with a 30 day warranty. In 30 days, they replaced it 2 more times. At 40 days it broke again, SORRY, only 30 day warranty. So he's not had a speedo since.

That's why I dislike those ratio boxes. They were mostly used on the NP203 & NP205 transfer cases in the 70's. If the speedo cable wasn't absolutely free running they failed pretty quick. The new ones are probably made from Floor sweepings over a charcoal furnace in China.
If you can do it with the drive and driven gears, or better still just the driven gear, go that route.
The tables in the parts books are easy to read. Then you install the needed gear or gears that cost peanuts and 10 minutes or an hour on the outside... Even if you have to swap the driven gear carrier and all the tail housing seals and the bushing on the tail housing you aren't into it for any $200. The shop that raped your dad should be ashamed.