My odometer is inaccurate. It didn't work at all until I fixed it last month with the DieselGiant method. I checked it against a known distance tonight and it turned out to be like reading 80% actual mileage. That scared me! At first I thought it was an engine problem!!! Actually my mileage is great!
I have heard that the W123 class suffers from this.

My '85's odometer is very accurate. Surprisingly so... 210K miles.

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1985 CA 300D Turbo , 213K mi

Just an idea. How is the condition of the speedometer cable?

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'81 MB 300SD, '82 MB 300D Turbo (sold/RIP), '04 Lincoln Town Car Ultimate

Sooner or later every car falls apart, ours does it later!
-German Narrator in a MB Promotion Film about the then brand new W123.

I think its binding because the speedo works funny too...

Have you taken it apart to look at the gears? When my plastic gears started failing, they seemed to be catching and releasing several times per second causing the speedo to jump. I assume it was causing the cable to "wind-up" and release. I disconnected the cable until I got home to fix the speedo gears because I was afraid it would break the cable.

I have the jumpy needle disease when im going under 20. I may have to measure what the distance somewhere with another car and check.

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91 350SD
14 F150 Eco
19 Fusion Hybrid
11 GT500

I have the same. Anyone know what causes this?

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Jumpy speedo needle is usually the result of the cable from the tranny running dry inside the outer cable. Causes the springy inside cable to wind-up and then unwind, often at a high rate of speed. Need to lubricate the cable, but others will have to suggest the best lube.

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1961 190Db retired
1968 220D/8 325,000
1983 300D 164,150

Interesting. My car does this, but it's on my 'low priority' list- is this lubing something that is easy to do?

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1984 300D Turbo Diesel - 211700+ mileage (now sold)
http://www.maj.com/gallery/Ned/benz/sig.jpg
In any of my posts, it is safe to assume
that I'm talking about this car.

I do not use original tire size and have gone for the tallest tire I can get under
my 123 body 300D.

This will cause my odometer to read under and my speedo too.

Add in my old dry speedo cable and it gets pretty bad.

I think I'll try the aerosal version of white lithium grease in the cable.

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80 300D 340K Owned 30 yrs
83 300SD 440K Owned 9 yrs - Daily Driver 150mi/day
02 Z71 Suburban 117,000
15 Toyota Prius 2600 miles
00 Harley Sportster 24k
09 Yamaha R6
03 Ninja 250