[AJC]

From hatzie PDF of Square Schematics & GM Training Manuals thread.

Originally Posted by hatzie

Useful Docs and Spreadsheets
What follows is an Excel 97 spreadsheet I put together several years ago to calculate the Speedo gears and engine RPM at various highway speeds. I was doing a "What-If" on transmissions, tires, and axle gear-sets and wanted to get transmission/transfer-case speedo gears as close to correct as possible. My 76 reads 1-2mph higher than actual speed. If my speedo reads 35 the local police dept's "Your speed is this" sign will read 33 or 34.

For Speedo gears you only need to edit the Tire Size & Axle ratio. The speedo gear table will automagically calculate the ideal Driven and Drive gears.
You then get to find which ones you can actually buy. You can edit the ratio adapter section to get you closer... If the gears you can actually buy are not close enough.

For the RPM tables you'll need to edit the transmission and Transfer case gear ratios as well. There are tables of ratios found in common trannys and transfer cases in the RH pane.

• RPM_Gear_Axle_Transmission_TransferCase.zip archive with Excel 97 XLS format and Libre Office / Open Office ODS format.

A Rockwell T221 should have a drive gear with 4 teeth and a driven gear option of 11, 12 or 13 teeth.

If you want I can do the math for the ratio adapter. I just will need the speedometer gear tooth count. Looks speedometer gear like the picture below.