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Old 11-28-2010, 07:43 AM   #1
towpainter3
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Red face speedo gears???????

Im gonna need to swap speedo gears im my d20. Im running 4.09 gears and 33's. Can some one possibly give me a clue on which gear I'll need and where I can get the lil critter. I've looked in the manual and mine doesnt have all the pages for the speedo gear selections. Any help would be greatly appreciated. Thanks brad
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Old 11-28-2010, 02:01 PM   #2
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Default Re: speedo gears???????

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Originally Posted by towpainter3 View Post
im gonna need to swap speedo gears im my d20. Im running 4.09 gears and 33's. Can some one possibly give me a clue on which gear I'll need and where I can get the lil critter. I've looked in the manual and mine doesnt have all the pages for the speedo gear selections. Any help would be greatly appreciated. Thanks brad
As you probably know, the speedo is driven off the transfer case adapter located on the rear axle output housing. And in the cable assembly you see a inline gearbox. Inside the gearbox are the components which are swapped around to calibrate the speedo readout.

All this information is in the part list for Scout II, the mt130. And...it's several pages of several charts with adapter gear codes (not part numbers) for each axle ratio vs. Tire size. The charts do not cover "modern" tire size designations, and only go up to "h (or hr) 78" series tires. And the "codes" would have to be crossed over to actual part numbers through an IH dealership I assume, though a speedo service center May be able to determine what they actually represent.

This type of speedo gear box was used on all 4x4 Scout and fullsize rigs since the speedo is driven from the transfer case. Only in the case of a 2x4 rig in either platform was the speedo driven by the transmission.

My advice...use a gps and plot the exact gps-displayed speed against your actual speedo readout. Then take that info to a speedo shop and have them advise to the correction factor and gear. Those speedo gearboxes are not specific to IH stuff but are very universally used in the automotive/truck industry.

The actual speedo "head" works off an industry-standard "turns per mile" calculation.

I don't know how or why what I have is possible (especially regarding old junkiron that set for 20+ years!!), as I have no record that the speedo system was ever delt with in going through an extensive service file. But the '73 t'all we've brought to life after all these years has a speedo system that is absolutely perfect in both speed display and in odometer ticks. The axle ratios are oem, but tire size (rolling diameter) is huge compared to oem. But from 0>85mph, the analog speedo readings are dead on accurate with four different gps units I have that can run simultaneously. Two of the gps devices are latest technology agps cellphones that can run several different add-on gps applications in multi-task mode. The third unit is a tomtom 910 nav system, the fourth one is a garmin handheld gps.

I've never seen this before regarding any other vehicle since I've been playing with gps since 1991.
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Old 11-28-2010, 04:12 PM   #3
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Default Re: speedo gears???????

Once again thank you. If im understanding you correectly the adapter on the tcase would be 1to1. The inline cable box is the change point. Luckily there is a speedo guy in town if he's not dead hell he was in business when I was a kid. I'll check with them and go from there.
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Old 11-28-2010, 05:03 PM   #4
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Default Re: speedo gears???????

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Originally Posted by towpainter3 View Post
once again thank you. If im understanding you correectly the adapter on the tcase would be 1to1. The inline cable box is the change point. Luckily there is a speedo guy in town if he's not dead hell he was in business when I was a kid. I'll check with them and go from there.
Don't think the ratio is actually 1 to 1. I have some of the pinion gears and they are all 13t. I have several of the plastic and steel gears that press onto the output shaft, they are different colors (different tooth counts) but I "think" they are for NP205. I have one steel gear which I know is a d20 but not what application it was on, several of these parts donor transfer cases I have came to me in tubs all ready broken down. The steel gear count I have is 5t.

The ihc parts list for sii shows two different shaft gear p/n's but no tooth count info.

Don't really matter...as long as your speedo pinion currently turns when it's supposed to. Your oldtimer speedo guy most likely knows this stuff off the top of his head by just lookin' at it cross-eyed!
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Old 11-28-2010, 06:27 PM   #5
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Default Re: speedo gears???????

After further inquiry, lookin at the cable, it seems to be a 1 piece design goin from the adapter on the tcase to the dash. Unless the lil box you describe is in the dash, I aint seen it yet I'll check with the speedo guy and see what he can tell me and let ya know this maybe a simple fix
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Old 11-28-2010, 06:45 PM   #6
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Default Re: speedo gears???????

No doubt there are some differences over the years.

There are many times another type of "gearbox" which is actually a speed sensor in the speedo cable buried down where the firewall meets the toeboard. That functions as part of some of the emissions packages. I've also seen the speedo cable ratio corrector gearbox in that area.

A 2x4 Scout II with a 727 had the speedo driven off the trans tailhousing, in the 4x4 727 that spot has the speedo drive adapter installed in the case but there is no pinion gear inside and it's capped off.

Some versions of the terra/traveler appear to have used an "extension" for the speedo cable also, obviously there are variations of the big picture out there, same for the fullsize stuff depending upon the drivetrain used.
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Old 11-29-2010, 07:17 AM   #7
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Default Re: speedo gears???????

The speedo correction gearbox that I have found on many IH's thru the years went between the cable and transfer case and was screwed on at the transfer case. I have a box full of good used ones all stamped with different ratios. If you can get me a percentage of how far off your speedo is I can see if one of the correction box's that I have will get your speedo matched up. Otherwise a speedo shop should have this item new and with the machinery they have they should be able to get you all fixed up.
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Old 11-30-2010, 01:42 PM   #8
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Thumbs up Re: speedo gears???????

After inquiring with the guy at the speedo shop. He explained how to get the info needed to figure out the percentage of difference in the speedo. Basically all u.s. Manufactureres set the speedometer and odometer to read 1000 revolutions per mile. The object is to figure out how many revs the offending cable is turning then ratio it backwards or forwards to get it to 1000 revs per mile. I've got to get this thing road ready and do the test.
This speedo shop does top end rebuilds and repairs for restoration guys. From what I saw at his shop he's damn good. Here's his website, there is a tech tip section that gives step by step instructions for figuring out the percentage of difference. He told me to be extremely precise in my notes and measuring. He said he could get the speedo correct within 1/2 of a tenth mile. auto instrument repair home page Bjorn sorenson/owner
423-468-3825
as soon as I figure out the difference and get him to look up the ratio in his manual, I'll give you a call and let you know which ratio I need. Thanks brad
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Old 09-01-2011, 11:01 AM   #9
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Default Re: speedo gears???????

Hey towpainter,
how'd it turn out w/the speedo?
Thanks for the link, I spoke to bjorn yesterday and as soon as mine is running I'm gonna give that 10 mile test a shot.
How much did it cost?
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Old 09-01-2011, 06:41 PM   #10
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Default Re: speedo gears???????

Hey man, after doin the 10 mile test, mine works out to be less than a 10th off. Im guessing after swapping cases from one with a 307 gear set to the 409 gears I have now and adding 33 in tires some where in the mix it seemed to work out.
After doin the test , I ran into a buddy with one of those gps thingys. At 60 mph my speedo reads 1 needle width under 60. Around here thats close enough. Hell most of the cops are just amazed its running at those speeds on the freeway!!!
Anyhow, bjorn is a really decent guy,but the little ratio boxes are pricy,Jeff at ihon told me he had a big box of them. After you figure out the ratio,give him a call and see what he has.
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Old 05-23-2016, 03:40 PM   #11
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Default Re: speedo gears???????

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im gonna need to swap speedo gears im my d20. Im running 4.09 gears and 33's. Can some one possibly give me a clue on which gear I'll need and where I can get the lil critter. I've looked in the manual and mine doesnt have all the pages for the speedo gear selections. Any help would be greatly appreciated. Thanks brad
http://www.tciauto.com/tc/speedometer-gear-calculator/
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