Speedometer adjustments

[HotRodPC]

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.
If you can do it with the drive and driven gears 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.

The claimed that was the set up on the machine the calculations, the part, setting up the part and retesting on the machine. Now days you don't need a machine. Just use a phone app or GPS to tell you how fast you're going and calculate it the difference, change gears and go for a retest drive.

 

[hatzie]

We have the GM parts books that I scanned to PDF... Much easier to just look it up. Back in the dark ages you just had to go to a local GM dealership.

If you would prefer not to download 500 meg of 73-91 light and medium truck parts and illustration manuals. Ian Harding has been good enough to post page by page images from the parts books in searchable form for GM vehicles and nameplates that became GM vehicles dating back into the 1920's. http://www.gmpartswiki.com
Ian has posted Jeep and Ford parts books as well.

You need four things besides the tables in the parts books.

1. model year
2. tire size
3. axle ratio
4. transmission or transfer case RPO

Here's a sample of the 1984 tables from the 52A 1979-1984 GM Light Truck parts books. They go on for many many pages before and after this page. The Military D series (M1008 M1009 etc GM CUCV) & the 73-78 & the 85-91 squarebody parts books & the 88-93 GMT400 parts books have these tables as well.
http://www.gmpartswiki.com/getpage?pageid=100140

You must be registered for see images attach

 

[HotRodPC]

There's also a formula you can use to calculate which gears you need. This is provided the speedo, which most are, I mile per 1000 revolutions of the speedo. On the Electronic Speedo's, the VSS is just a counter and your DRAC does all the math so you change your DRAC chip or you can even modify your DRAC with Dip Switches and make changes pretty much on the fly if you wanted to.