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Speedo Questions...

404 Views 4 Replies 3 Participants Last post by  Jim_Lou
Hello Forum,

i am having ratio problems with a o.e.m. Cj speedo i rebuilt not to long ago. i drove about 2600 miles at 70 mph, but the speedo reads about 1700 miles and 45 mph.
The unit was with a D-300, and is now matched with a D-20 tranfer case. Do they have different drive gears for the cable? The needle is smooth, so i'm thinking the cable itself is fine. i have the original D-20 speedo for parts.
Basically, are all Cj speedos the same? Then the problem would have to be in the transfer case driving gear. Or my rebuild. Is there a simple fix. Thanks in advance. Always lots of help available here.

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There's kind of a standard for mechanical speedometer drives of 1000 RPM at 60 MPH, or 1000 revolutions per mile. I suspect that all recent CJ speedometers would be calibrated to that standard, but can't say for sure.

The drive gears in the transfer case need to be matched to the axel ratio. I suspect that the gears in the D20 case are matched to 4.11 or 4.56 axels, and you've got it in a Jeep with 3.08 or 3.26 gears.

Your reading is about 65% of actual distance (1700/2600=.654). 3.08/4.56=0.675.

I don't know if the speedo gears are interchangeable between D300s and D20s. If so, swap them. If not you'll probably have to go to a speedometer shop or Jeep dealer to get the right gear.

If I'm all wet on your axel ratios, there is a serious problem with the speedo, but since both speed and distance readings are off by a similar factor, I think the problem is in the drive mechanism.

As a test you could disconnect the speedo cable from the instrument and wrap a flag of tape around the end. Then slowly drive a measured quarter mile while a passenger counts the turns of the cable. It should be close to 250. If it's more like 160, you've found the problem.
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In reply to:

Will i be able to just put in a suitable drive gear to make up for the axle and tire ratio changes? Is this a common fix at any speedo shop?

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Yes, within limits, and yes.

There are drive gears for any axel ratio that was ever factory installed with that transfer case, but not exactly. For example, there's only about 6% difference between 3.32 and 3.54, so they might use the same speedo gear.
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