shortcut to answer if anyone is playing along and doing the math. why we don't think of these the easy way i don't know. but here it goes the short followed by long.

with the following assumptions
3.09 rear end
Stock 7 tooth drive gear
Stock 20 tooth driven gear
1000 (really 1001) rpm for a mile / mile per hour speedometer
Then the OP will need a 1.34 to 1 Steward Warner type of adapter. if they even made this ratio. I have seen a lot on ebay less than 1 and more than one 1.15 and a 1.20 ish.
1.34 is on the money But something close will be really close on the speed with 1000 RPMS per mile

The basic math.
Stock rear end ratio on the truck and how the speedo gears are set based on the parts and manuals is 4.11 and 7 and 20 and 1000 of course
He is running a 3.07 if everything is stock its as simple as 4.11 divided by 3.07 or 1.34 (1.339 rounded)
why i did not think of this moons and days and what 8 pages of post ago I don't know.
Probably because we needed to validate rear end and all the other items anyway in case someone had changed something.

The long math

5280 feet x 12 is 63360 inches divided by PI (3.14) 20128 divided by tire diameter 29 (for easy math) 696 Revolutions per mile.
696 x 3.07 or 2137 drive shaft rotations per mile.
2137 divided by 2.86 (the stock 7 drive and 20 driven ratio) or 747 RPM per mile to output to speedo which is why he is off.

1001 divided by 747 is 1.34 (1.33868 if using 1000) ratio needed to increase the cable back to 1001 rpms ( why 1001 say, well you need to full ration on each count so you add one but since its so miniscule in calculations most people just use 1000 to keep confusion down)

And therefore 1.34 to 1 gear ratio adapter that fits the speedometer cables.

FYI the 4.11 does match the math calculations as well with the 7 and 20 teeth gears when doing the math as well with stock items. and 29 tires to validate my theory.

As long as the tires are close to 29 inches in diameter of course. change the tire size and have to change the adapter or something else of course.


One Note the actual driven gear teeth would be 15 with all the items above if you were to replace the stock 20 gear to get to the correct ratio without a adapter. But I don't believe a 15 gear is available for this transmission and speedo type.

Last edited by hcb3200; 05/09/2024 1:08 AM.