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I've never had reason or a good enough reason to work on the two speed speedometer drive unit of these units that have the two speed axle as I presume your truck is equipped with but I expect that is the problem. The little two speed ratio change unit to keep the speedo correct with the speed of the axles is not functioning properly. GM used to use a vacuum system but I am guessing went to electric shift and then electric for the speedo box as well as time went on.

That's what I would speculate is the issue and a clue into that is sometimes erratic readouts of the speedometer depending on if one is in high or low range of the diffs two speed, another words no change in ground speed but shift it either up or down and note a difference in speedo reading. That or just remain in one speed within that box and most likely the low range speed so when its put into high range it would show too fast or at least I think that's how it would behave
 

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It could be what the guys said above for sure. We had o c70 at one time too, and it came with 10 - 20 tires. We put the bigger 11R-22.5's on it and had the same problem. If the truck came from the factory with 10-20's and you or someone else put on the bigger tires on, it changes the reading of the speedo. You can get a inline speedo correction box that installs on the speedo cable but it was a few hundred bucks, so we passed on it.
 

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I was always under the impression that the 10.00 x 20 tires were very close in diameter to the 11R-22.5 although I've never looked at a tire revolutions per mile chart to compare and come to think about it we don't have one 22.5 tire on this yard. I do know that some tire brands can have a marked difference in diameter which of course poses all sorts of potential problems in matching up duals and would throw off a speedometer.

Having said that, if larger tires are mounted up, the speedometer will read LESS for a given ground speed because now the driveshaft turns less revolutions for the amount of forward travel in distance the truck would move along the ground. So for arguments sake if this trucks speedometer reading was correct before, it would be a smaller diameter tire that would cause the effect of it now reading too high for the actual speed traveled.
 

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It could be what the guys said above for sure. We had o c70 at one time too, and it came with 10 - 20 tires. We put the bigger 11R-22.5's on it and had the same problem. If the truck came from the factory with 10-20's and you or someone else put on the bigger tires on, it changes the reading of the speedo. You can get a inline speedo correction box that installs on the speedo cable but it was a few hundred bucks, so we passed on it.
Those tires are the same diameter, same as 10R/20s, as a matter of fact you can run them together with 11.00R/22.5s don't mix radials and non radials though. 10R/22s and 11.00R/24.5s are also the same diameter.

I go with Northern Farmer, your speedo is stuck in high. How are you determining your going a hundred? Phone app? Maybe your two speed axle is stuck in low and your speedo is switching only.
 

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You guys are right on the size, but I went from bias and near bald 20's to 22.5 recapped deep tread, and side by side, tires were quite a bit bigger. And yes the speed went the other way, read less than actual speed.
 

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Easy way to test if the speedo drive is shifting, is simply shift the two speed under load up and down in any forward gear. The speedo drive will shift even though the axle can't. There will be noticeable change in the speed indicated if it is working.

It is possible for one to shift and not the other, as in the drive and the axle. One of my trucks has developed a quirk where in cold weather when ever I shift into one of the forward gears in the pattern, the drive will shift but the truck doesn't. When I shift again, the speedo returns to normal. I haven't tracked it down yet, but if I pull on the cable running down the shifter, I can make it do it intermittently. First place I will be checking on mine is the switch.

As said, 10.00 20s are the same as 11 22.5s. I run them mixed on various farm trailers.
 
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