RPM-Speed Relationship

[OkieTruckerD]

Yes, i did change the gear in the tailshaft, white in color, it's been a while, but i think the tooth count on the gear is 16 teeth

 

[OkieTruckerD]

This also may contribute to my cruise control issue....posted VERY recently

 

[Brianedwardss]

Yes, i did change the gear in the tailshaft, white in color, it's been a while, but i think the tooth count on the gear is 16 teeth

I ran your truck through another formula I've got written down. I may have found the issue. As stated, you've got a white driven gear, and your speedometer is slow.

According to my saved notes, White is 17 tooth. My notes read as follows:

Ford Manual Transmissions:

16-tooth-wine (C0DZ-A)
17-tooth-white (C3DZ-C)
18-tooth-yellow (C0DD-B)
19-tooth-pink (C0DZ-B)
20-tooth-black (C1DZ-A)
21-tooth-red (C4OZ-A)

To find what driven gear you need for your truck, its a bit of a process. Most speedometers need 1000 rpms per mile to operate correctly. I'm assuming that these IDI's are similar. To find what driven gear you'll need, here's a formula

1000 speedo RPMs = (20168 X Diff Ratio X Drive Gear Teeth) / (Tire diameter X Driven Gear Teeth)

The variable is gonna be the drive and driven gears. We want the driven gear. Most ford transmissions I've dug into had 7-tooth drive gears (E40D being the exception). So...

(20168 X 3.55diff X 7DriveTeeth) = 501,174.8

(31.59 TireDiam X 17DrivenTeeth) = 537.03

501,174.8 / 537.03 = 933.23 Speedo-cable RPMs at 60 mph. That equals about 6.7% slow. I know from prior experience that a slow speedo needs a smaller driven gear to correct, because a smaller gear will turn faster.

So I plug in a Wine 16-tooth gear, which is the smallest you can buy that I'm aware of:

(20168 X 3.55 X 7) = 501,174.8 (31.59 X 16) = 505.44

501,174.8 / 505.44 = 991.56. That's less than one percent off from the 1000 RPMs your speedo needs to be accurate.

Looks like a 16-tooth-wine (C0DZ-A) gear will do the trick.