Speedometer registers 5mph fast
 

Hi,

My 1983 300d has a speedometer that registers 5 mph too fast.
The odometer works perfectly.
The tires are the recommended size for the car.

Any idea what may be causing this. It currently has 83K miles on it and it has always been this way since my ownership of 17K miles.

Thanks.

Normal. Mechanical speedometers were never accurate.

Your 83 300D should have the 3:07 Differential.
To check it, look at the Differential from the rear.
lower left corner scrape off the grease/grime and
there is a set of #`s stamped.

The Spedo has a part # also 123 xxx xxx xxx
use to have a list of what #`s went with what Diff.
Some one may have it.
The Diff and Spedo are a set that go together.

Yeah could just be off because of being mechanical.


Charlie

My 83 300SD does the same

Search function.

http://www.peachparts.com/shopforum/diesel-discussion/238488-backyard-speedometer-adjustment.html


http://www.peachparts.com/shopforum/diesel-discussion/230116-any-way-calibrate-w123-speedo-2.html

That`s a few years back, forgot about adding to these posts.
Haven`t taken a spedo apart in a few years. they are pretty simple.




Charlie

If you have a 1985 speedometer in a 1983 car then it will show 5mph faster at around 70MPH.

1985 cars had a 2.88 axle and your 83 should have a 3.07

Easy to tell which speedo you have by looking at where the 3 dots are:

3 dots at 77mph - 2.88 speedo

3 dots at 72mph - 3.07 speedo

Oh, you good people make me laugh and I appreciate it!
After decades of working on 617s/W123s, I *still* learn things...

See the attached picture of the original speedometer from a 1985 300D.

Yep, love this forum. Who knew? I am headed out to look at my speedo in the '81 300SD.

I don't know where you're located but most major towns will have a speedometer shop - in the Detroit area it's Bob's Speedometers. They'll have some kind of chassis dyno so they can roll the car up and do a speed correction. Generally they'll make a small speed correction box to go between the trans and the speedo cable - problem fixed. At the worst they'll make you a correction table to post near the speedo but I've only seen that happen once at Bob's. BTW - when I worked at the GM Proving Grounds all of the test vehicles had that correction chart stuck to the speedo so us test drivers could drive at the actual speeds for the test cycle and not the indicated speed so they KNEW their speedos were off!

Dan

It depends if the car is a CA model with the 722.4 or Fed model with 722.3 with 1.48 or 1.44 3rd ratio
I have two 2.88 speedos, one with three dots at 74mph other at 77.

Good point but the 722.4 was only installed on CA models and only in 1985 so it's a rare speedo but at least we know that it has 3 dots at 74

To be clear: My above referenced "1985 300D" is a CA model and indeed, has the factory installed 722.4XY.

I know this sounds weird, but the speedometer, trip odometer and the odometer are all separate instruments, geared off the one speedometer cable. If you've verified that the trip and regular odo's have the correct reading, then the instrument is correct for the car, and well matched to the tires and diff. That means that the only problem is that the speedometer is reading low. That can be easily adjusted by pulling the needle and repositioning it slightly. Before you begin, use a short length of speedometer cable to drive the speedometer with an electric drill. Not the speed reading. Then just adjust the needle so that it reads 5mph faster.