1980 CJ7 speedometer slow - JeepForum.com
 
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post #1 of 9 Old 06-15-2019, 07:53 PM Thread Starter
clarkg
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1980 CJ7 speedometer slow

I recently bought a 1980 CJ7 with a 4.2L and T176. The speedometer runs really slow. At 60mph it registers about 45.

The odometer seems to be registering correctly.

Should I take the speedometer apart and try to clean and lubricate it?

Any ideas on what to look for?

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post #2 of 9 Old 06-16-2019, 04:59 PM
popascj7
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Have you checked to make sure you have the correct speedometer gear installed for your gear ratio and tire size?
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post #3 of 9 Old 06-16-2019, 05:44 PM Thread Starter
clarkg
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Quote:
Originally Posted by popascj7 View Post
Have you checked to make sure you have the correct speedometer gear installed for your gear ratio and tire size?
No I havenít checked this. I did check the odometer against mm posts and it seems correct.

Just thinking that if my odometer is correct the problem must be that there is too much friction in the speedometer needle pivot.

Is there an easy way to check the gear ratio?
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post #4 of 9 Old 06-17-2019, 02:12 PM
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There are a couple ways to determine gear ratio. First would be to count the number of teeth on the ring gear and pinion then divide them. The other way that doesn't require opening the diff would be to spin the drive shaft with the Jeep in the air and count the number of times the driveshaft revolves for every complete revolution of the tire. If the tire makes 1 complete revolution and the pinion spins 3.5 times, you would have 3.54. If the pinion spins a little over 4 times then you have 4.10 and so forth.
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post #5 of 9 Old 06-18-2019, 06:55 AM
bob4703
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If the odometer is right and the speedometer isn't then the gear is good or at least isn't the culprit. Both run from the speedometer cable. Sounds like it is time for a new speedometer assembly or a rebuild of the present one. Good luck!

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post #6 of 9 Old 06-20-2019, 06:13 PM
80cj
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Quote:
Originally Posted by bob4703 View Post
If the odometer is right and the speedometer isn't then the gear is good or at least isn't the culprit. Both run from the speedometer cable. Sounds like it is time for a new speedometer assembly or a rebuild of the present one. Good luck!
Mine went south a few days ago also. Ordered an Omix one. I'll find out how good it is.
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post #7 of 9 Old 06-20-2019, 06:44 PM
John Strenk
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I think the ratio of 1000 rpm :60 mpg is what most speedometers are set up for.

If you have a 1000 rpm drill you can check the accuracy of the speedometer.

You might be able to adjust them somehow. it's just a spinning magnet in a cup (Or maybe the cup is the magnet) held back by a spring.


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post #8 of 9 Old 06-20-2019, 10:59 PM
80cj
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Quote:
Originally Posted by John Strenk View Post
I think the ratio of 1000 rpm :60 mpg is what most speedometers are set up for.

If you have a 1000 rpm drill you can check the accuracy of the speedometer.

You might be able to adjust them somehow. it's just a spinning magnet in a cup (Or maybe the cup is the magnet) held back by a spring.
John, I was meaning to ask you about the workings of a speedo. So 1000 RPM=60MPH? I don't know what the heck is wrong with mine. It really lags as I accelerate, then as I settle in at a steady speed it seems to catchup.
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post #9 of 9 Old 06-21-2019, 05:22 AM Thread Starter
clarkg
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I was wondering about cleaning and lubricating the mechanism that drives the needle. Since my odometer seems to be working, i am thinking that the needle must not be moving freely and thus is prevented from traveling as far as it should.

Wondering if anyone else has tried this?
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