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Speedo reads too slow.

A

alex33072

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I put together a M1009 from parts obtained from various sources. The transmission came from a M1008, I think. After looking through some manuals it appears there is some sort of regearing box that the speedometer cable plugs into rather straight in the way I have it. Is this correct? Am I missing something else? Anyone have one or know where to get it? Thanks,
Alex
 
richingalveston

richingalveston

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that is a gear box for the cable. the 1008 has 4.56 gears and the 1009 has 3:08. you need the gear box if you still have 3:08 gears in axle. The gear box corrects the speedometer for the different rear end gears. If your t-case has the box then it came from a 1009. the tranny could still be from 1008. The tranny and transfercase are the same in the 1008 and 1009 with the exception of the gear box on speedo cable
 
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Chaski

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If you don't want to cheat and use a correction box you could just replace the driven gear on the transfer case end of the cable.

There are a couple different drive gear tooth counts, and there are two different driven gear holders with different offsets to work with the wide range of driven gear choices. You might want to pull out the assembly from your transfer case to see what you have, then use this calculator to see if you can get it correct with swapping out the driven gear on the speedometer cable. Sometimes you have to swap the drive gear to get the ratio right, and if that is the case you have to pull the back of the transfer case apart. Anyway, take a look, it will be cheaper and cleaner if you can just swap a gear or two.
http://www.73-87.com/7387garage/drivetrain/speedo.htm
 
Sharecropper

Sharecropper

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Chaski said:
If you don't want to cheat and use a correction box you could just replace the driven gear on the transfer case end of the cable.

There are a couple different drive gear tooth counts, and there are two different driven gear holders with different offsets to work with the wide range of driven gear choices. You might want to pull out the assembly from your transfer case to see what you have, then use this calculator to see if you can get it correct with swapping out the driven gear on the speedometer cable. Sometimes you have to swap the drive gear to get the ratio right, and if that is the case you have to pull the back of the transfer case apart. Anyway, take a look, it will be cheaper and cleaner if you can just swap a gear or two.
http://www.73-87.com/7387garage/drivetrain/speedo.htm
Click to expand...
To follow-up what Chaski said, every transfer case has a Drive Gear and a Driven Gear to produce the correct speedometer cable revolutions-per-mile from the transfer case to the speedometer. The Drive Gear is installed when the transfer case is assembled at the factory and cannot be changed without dis-assembly of the transfer case. The Driven Gear can be changed easily by simply removing the sleeve and changing the gear. The only thing to be careful about is that there are 2 different sleeves, 1 for the lower tooth count gears and 1 for the higher tooth count gears. All this is explained in the link in Chaski's post above. According to my research, most all NP208 transfer cases originally furnished in the M1008 and M1009 came with the gray 15-tooth Drive Gear, GM part #8642620. If this is true for your transfer case, then the only task at hand is to determine which Driven Gear you need to match the cable RPMs with the rear end gear ratio and tire diameter. Once you do the calculation, then it is a simple purchase and installation of the correct Driven Gear to achieve correct speedometer readings. A handy calculator is at http://www.bgsoflex.com/speedo1.html . The Speedometer Revolutions Per Mile should be 1000; then enter your Tire Diameter, then Differential Gear Ratio, what you want to find (Drive Gear or Driven Gear, in your case, Driven Gear), Drive Gear tooth count (in your case 15), then click Compute and Bingo! The Driven Gear tooth count will be displayed. Then go to the chart on Chaski's link and obtain the part number for the correct Driven Gear you need and then go to our favorite online auction site and do a search for "NP208 Speedometer Gear". Buy it and install it and you should be good to go. Pay attention to which sleeve you have and make sure you use the correct sleeve for the Driven Gear you choose. All this information is in Chaski's link. Also remember to thoroughly grease the Driven Gear shaft when you install it into the sleeve. If you still have speedometer fluctuations after doing this, you probably need to replace the 30+ year old speedo cable.

Hope this helps.
 
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