Speedometer help

[Massboy]

I'm working at getting my speedo right in my truck. I've got the gray drive gear in my 208tc and the driven gear is the green 42 tooth. The 208 was from my donor Jimmy which had 3.73's I think and 33's. When I have the speedo hooked up with my 3.08's and 32 BFG tires I get 7mph below what I'm going. When I looked at the 208 I took out of my M1009 it has an increase adapter on it. When I put it in line the speedo now reads 5mph over at 55mph. The 208 from my M1009 has the yellow 41 tooth gear so I tried that. Without the adapter I showed 7mph slow and with, 7-8mph too fast.

I assume that the adapter is a increase adapter as the MPH goes up when I use it. Does anyone know what the ratio of this adapter is as supplied on a M1009? One tooth difference with the gears seems to be 2mph. I got closer with the larger gear so I'm wondering if I go to the 44 tooth gear will I get what I need or am I better off with the 43? I do not want the speedo to register faster MPH than what I'm going as I will be adding to the odometer needlessly. Any ideas. I have looked at this site http://www.transmissioncenter.net/speedometer_calibration_______va.htm#gm ratio adapter and will call them but didn't know if anyone knew the adapter's ratio.

Reading their site if I don't use this adapter I would need a 28t gear which doesn't exist for the 208tc.

 

[Massboy]

I think I figured more out. The Stewart Warner adapter has a 1.300 on the outside which I understand is the ratio. When determining the proper gear it is based on 1000rpms to the speedo. This would normally mean the gear output should be 1000rpms when the correct gear is used. Using the 1.3adapter I took the 1000rpms and divided by the 1.3 giving me 769 rpms.
769 is the input at the adapter to give it a 999.7 output to the speedo.

Using the computation charts available I substituted the 1000 speedo rpm with the 769rpms. Entered my tire diameter (31.7), drive gear in the tc (15t) and the ratio (3.08, had to be entered as 3.10) and came up with a 38t driven gear. I believe this is correct. Hopefully if anyone has an issue down the road this might help them.

Bowling's Speedometer Calibration Program

Input Parameters Are the Following:

• Speedometer Revolutions/mile = 769
• Rear Tire Diameter (Inches) = 31.70
• Differential Gear Ratio = 3.10
• Drive Gear Teeth Number = 15

Computation Results:

Computed Driven Gear is 38
Ratio of Driven Over Drive Gear is 2.5

 

[RootBreaker]

all i know is i had a thread on this... and took mine apart.. drilled the rivets out.. and i even found someone. they said

drive 1 mile then 10 miles via GPS and give them the reading of mileage on the speedo and he would fix it. gears in there can be swapped...