Best way to correct speedo/odometer error
 

The speedometer on my signature car has always read about 6% fast. Recently, I checked the odometer against my truck, and it is also fast by about the same amount. Before I purchased the car, the transmission was replaced with a MB rebuilt unit. Dividing 2.88 by 3.07 (the current axle ratio) gives me about the same error (.938). I would suspect that the transmission has the drive gear for the 2.88 ratio. What is the best (or easiest) way to correct this error?

All three of my W123s had spedos that read fast and changing to 195/75 tires helped allot.
I used my GPS when I was monitoring these millage's. Above 60 mph it starts to become noticeable that they are reading a little high but nowhere near 6% even at 75-80 mph.

There used to be little gearboxes that could be inserted into the speedometer cable. Depending on the gears, the "data" was shifted faster or slower. If such things are still available, your error would be greatly reduced or eliminated.

Jeremy

The MB out put on the transmission is 1:1. the Speedometer is geared to the Differential, not the Transmission. Look on the bottom left corner (Looking from the back to the front) of the Diff. scrape off the dirt and grime and some numbers will appear. the last 3 will be the gear ratio of the Differential.

The Differential may have been changed at some point and be correct for the Speedometer.

Charlie

What size tires are you running?

Have a Question: my 1983 300sd (w126)has an error in the speedometer only
if you can trust a gps the odometer agrees within 1% so that's pretty good.
still running stock size tires. and have seen the electronic adapters that boost or reduce the pulses coming into the speedo housing but it seem's that the would also affect the odometer readings also, has any one imagined a way to compensate for the 5Mph error at 50Mph?

The difference in speedometer error between a 3.07 and a 2.47 is about 24%.

A 2.88 speedo will be marked W1155 on the back

A 3.07 speedo will be marked W1330

It does indeed have the 3.07 ratio. I verified that when I did the axles. If I understand you correctly, then all the transmission drive gears are the same, regardless of the final drive ratio. I presume then, that the correction could be made at the odometer drive gears.

Stock tires, 195/70-14's.

rscurtis, if you can confirm that you have a 2.88 ratio speedometer now, then swapping it for the more common 3.07 unit would fix the discrepancy.

Get your GPS and make a list... Road speed and Speedo reading in increments of 5 mph. I had to do this with my 85 300D. It is generally 6-7 mph +- off. For example if I want to go a legal 35 mph the speedo has to be at 41 mph. What I did is make a strip which I taped to the bottom of the cluster assembly so I can make sure I'm legit.

This is a lot less expensive than changing the tires, or doing something else which costs you bucks. (I've checked the speed with 2 different GPS units and the random radar stationary sign and they all indicate the GPS readings are correct.)

I'll check next time I remove the cluster, but have no reason to suspect that the speedo has been changed, as the vehicle's mileage history matches Carfax. I do have an '84 speedometer, but the interface for the cruise control is different. I just want to confirm that the problem is NOT a drive gear in the transmission, but can be corrected at the speedometer. I suspected the trans. because I know it has been changed.

I have an 84 300D speedometer with the number 123 542 28 57 on the face.
The 84 uses a 3:07 Diff like your 83 300D.

The 85 300D with a 2:88 Diff has the number on the face of 123 542 27 57.

If you Odometer and Trip are spot on but the Speedometer needle is off, (fast or slow) you can take the needle off and reset it on the shaft.

Lift the Needle up over it`s resting pin, and see where it now rests. there is a small white mark on the edge of the face. this is a reference mark to where to remount the Needle when taking the unit apart.

There is a small spring on the shaft, so moving the Needle right or left of this mark will give the spring more or less tension.

Reposition the Needle, lift it back over the resting pin and you are back in business.

Charlie

The 1983 speedometer part number on the face is 123 542 18 57 and it is a 120 mph speedo.


The 1984 speedo should have the same cruise control interface, a little back VDO box with the two prong electrical connection on the back.


.

Think that is referred to as a speed Sensor. Most all 123`s seem to have it.

Charlie