Speedometer Correction

I know newer stuff you can switch the speedometer gear at the transfer case for different ratios. How was it done on the Dana 18? Secondary gear box?

 

You can change the speedometer gears in the transfer case or get a ratio adapter that connects in-between the transfer case and speedo cable. Another option if you dont mind the expense is a gps speedo gauge from speedhut.

 

This post on the forum has a chart of the available Dana 18 speedometer gears: Model 18 Speedometer Gears

Note each section is a series of gears. Even though the last two sections have 6 tooth drive gears, the pressure angle is different and you can only use the driven gears in the section matching the drive gear.

You will need to know what drive gear you currently have to see what driven gear options are available. If you pull out your driven gear, it should have numbers to match it to the chart and tell you what your drive gear is.

On my '71, I would have needed to change the drive gear and did NOT want to take the back of the transfer case apart. I went with an external gearbox instead.

You can find gearboxes on ebay. Search for "speedometer reducer" and look for the ratio you need. I found mine by looking up a part number for the ratio I needed in this chart: http://www.clarkbrothers.net/pdf/SC795_Catalog2_Page_5to7.pdf. I found a matching gearbox on ebay for $30.00.

When the gearbox showed up it had a keyway output and didn't come with the square drive adapter. I ordered one of those. GM Part # 88961013. Also available on ebay: https://www.ebay.com/itm/331379419165

The gearbox I ordered was too bulky and interfered with the parking brake backing plate. I used a short extension cable to solve that: https://www.summitracing.com/parts/...MI-9LAo4GJ-AIVAx6tBh1v0QX0EAQYASABEgJfNPD_BwE. Other gearboxes may clear without needing this.

Here's a post on my build thread with pictures of the whole affair: 71 Wheeling Rig

 

Thanks you! This clears it up. I think I'll go the gearbox route.