I bought a 1980 300TD wagon about six months ago. The previous owner warned me that the speedometer was a little off, I just didn't know how bad it was. I searched to see if anybody else has had this issue, but I can't find anything. The speedometer is inaccurate at all speeds. I've done a few tests with roadside speed displays and I usually just drive about as fast as the cars around me to estimate my speed.

Speedo / Actual speed:
10 mph / 10ish mph
20 / 25
25 / 35
30 / 42ish
35 / 50
40 / 60ish
45+ / fast

The speedometer caps at 45 mph, but I'm going way over 60. I've been ok so far because I drive mostly in the city where there are other cars to compare speed with, but I know I'll get unlucky one day and get a fine.

The odometer works, I've tested it against gps. The previous owners worked on the car some, so it's a possibility that the instrument panel has been replaced. I'm not sure if the problem is with the speedometer itself, or the cable. Maybe there is an issue with springs or worn out parts in the speedo. Or something else?

What do you think? Let me know if you need any more information.

In the past someone has probably replaced the speedometer and replaced it with the wrong one. Mercedes W123 cars used several different speedometers depending on the engine and especially the rear axle ratio. Even if you have a speedometer that says 125 MPH as the maximum speed, there are still variations in 125 MPH speedometers, for example a 1985 W123 diesel car uses a different speedometer than earlier years despite the fact that prior year speedometers (1983 & 1984) look identical.

The best way to determine if you have the correct speedometer is to look at the part number of the speedometer at the bottom of the speedometer face. Call the dealer and ask for the correct part number for the speedometer for the year and model of your car, you will probably see a difference which explains the inaccurancies of your speedometer.

Unfortunately the only way to correct the situation is to replace your speedometer with the correct one with the correct part number.

Hmmm, I bet the first sign that the speedometer was replaced is that it only goes up to 100 mph. I'll have to sniff around and try to see what the correct part is.

This is the number on the speedometer, if anyone is interested:
123 543 30 01

But what would cause it to stop moving over 45? Even the wrong speedo would show more than that surely?

__________________
1978 300D, 373,000km 617.912, 711.113 5 speed, 7.5mm superpump, HX30W turbo...many, many years in the making....
1977 280> 300D - 500,000km+ (to be sold...)
1984 240TD>300TD 121,000 miles, *gone*
1977 250 parts car
1988 Toyota Corona 2.0D *gone*
1975 FJ45>HJ45
1981 200>240D (to be sold...)
1999 Hyundai Lantra 1.6 *gone*
1980s Lansing Bagnall FOER 5.2 Forklift (the Mk2 engine hoist)
2001 Holden Rodeo 4JB1T 2WD

The diff may have been changed too.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

That's confusing too! Maybe a spring is stuck or something. I'll look for a correct speedometer and see if the problem goes away.

Here are some good links to read through on Spedo repair.

Repairing the odometer on W123 | OldMerc.net

Odometer Repair W123

This following one he mentions to use a utility knife to score the shaft.
I found using a pair of 6" needle Nose VG put the ridges in the shaft to
keep the Pot metal gear from slipping.
Good write up though.

https://www.ifixit.com/Guide/Repairing+Mercedes+W123+Odometer/25421
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there were three HP ratings on the OM616...

1) Not much power
2) Even less power
3) Not nearly enough power!! 240D w/auto

Anyone that thinks a 240D is slow drives too fast.

80 240D Naturally Exasperated, 4-Spd 388k DD 150mph spedo 3:58 Diff

We are advised to NOT judge ALL Muslims by the actions of a few lunatics, but we are encouraged to judge ALL gun owners by the actions of a few lunatics. Funny how that works

To fix your speedo you need to remove it from the dashboard .Take the speedo head out and when you have the head in your hand lift the spointer over the end stop anticlockwise if that makes sense .Then mark the possition that the pointer rests at with a pencil .Then remove the pointer from the speedo and re fit it again 1/2 the distance from pencil mark and end stop .So if you will be replacing it closer to the end stop .Then lift it back over the stop. It may take a few times to get it right ,but you might be lucky the first time,like i was . To give you more idea check out DieselGiant web site for speedo repair. Thinking about this you may need to go the other way and put tenssion on the spring .Its trial and error at this stage,

All of those links describe odometer repair. Although the speedometer and odometer share a cable and live in the same box, the are different instruments.

If I'm not mistaken, the top speed shown on the speedo was limited by Federal regs in 1980. I could be wrong, but there was a period where this was the case. So it may be correct for the car.

If it was mine, my first shot would be to sync the needle at 60 mph. Lift the tip of the needle over the stop, but don't remove it. Note the position, then remove it and reinstall the needle a few degrees clockwise. See what you have, repeat as necessary. Then observe the error across the whole range.

The speedometer portion of the instrument consists of a rotating bar magnet, which rides inside a steel cup. The cup is mounted to the needle shaft. As the magnet rotates, it drags the cup. The faster it rotates, the harder it tries to rotate the cup. There is also a hairspring on the shaft. Calibration can be done by either replacing the hairspring or adjusting the strength of the magnet, neither of which are easy DIY projects. However, the shaft rides in a brass bearing, and sometimes a tiny bit of oil on the bearing is all you need.

I don't know of an online reference for VDO speedometers, but the principles are the same for Smiths, so this illustrated reference will at least give you an idea of how they work:

http://triumph.daveola.com/NOTES/Speedo_Repair.pdf

Yup, the memory hasn't failed yet. 1980 was the year of the short speedometer:

85 MPH Speedo