Originally Posted by hatzie (Post 5621043)

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.

Originally Posted by Verbal (Post 8999268)

I like the chart at this link. https://speedometercablesusa.com/gear_box_adapters.html

I have no personal experience correcting speedometer though (yet)

Originally Posted by AJC (Post 9005041)

You will need a correcting adapter with a ratio of 1.281779661 or as close as possible. Now if your 33" tires run +/-0.5 of an inch, you could still see up to a 3% error. Which would be +/- 2 MPH at 65 MPH.

Attachment 2150855

Here is a GoogleDoc I made that you can play around with tire size and gearing. The formula is from Speedometer Cables USA under "Need a correcting adapter".

CORRECTING_ADAPTER_CALCULATOR