W116 Speedometer

[The Warden]

Hello!

I've made reference to this before, but it'd be nice to get a definitive answer if at all possible.

Can anyone explain the speedometer system on the W116 chassis? I know it's a mechanical system, and I'm guessing that the cable is hooked to the rear end of the tranny. Is this correct? How is the speedo calibrated for differential gear ratios? Can just the driven gear on the speedo be changed, or does the drive gear in the tranny need to be swapped as well? Or is the calibration in the speedo gauge itself (like it is with at least some electronic speedo cars such as the 126)?

I ask because I'm thinking on the lines of trying a different differential gear ratio in my 300SD (want to go down to 2.47 for fuel economy purposes), and it'd be nice to keep the speedo reading accurately.

Thanks in advance!

[minger]

Well, from when I was chasing what I thought was a broken speedo cable, I do rememeber it attaching to the tranny right before the drive shaft began.  But was on a 450SEL.  Don't know more than that becasue I was just cable chasing.  Hope that helps and good luck

[AMG69]

2.47? I thought the diesel would already be very economical and isnt 2.47 going to make a slow car REALLY slow?

[Tomi]

This is from my (bad) memory: the speedometer has a rotating cup and on the other side a disk with a spring. the speedometer needle which is attached to the disk moves according to a magnetic field of some sort as the cup rotates faster. I dont think you can alter that yourself. But there are speedometer shops that can calibrate the speedometer to different values.

[116.025]

From what I've seen, a good option would be to get an adapter made that you would place between the end of the cable and the speedometer, which I believe Lauderdale Speedometer does that sort of thing.

I know when I got my 280SEL and it had a speedo from a 450SEL, it gave some interesting readings...

[Papalangi]

If it works like an domestic (US) car, the gear on the trans end of the cable (driven) can be changed to calibrate the speedo.  The gear in the trans is the drive gear and is fixed to the output shaft.  The speedometer does not need to be changed unless it has gone out of calibration which would lead to a nonlinear display rather than a fixed error.

I've seen two types of errors on my cars.  One is from incorrect tire sizes and is a percentage error, usually 10% or so.  The other, which is what's happening with my 450SEL, is that it reads 5 MPH too high across the scale.

Michael

[The Warden]

2.47? I thought the diesel would already be very economical and isnt 2.47 going to make a slow car REALLY slow?

A few people have done this on 126 300SD's...and they found that acceleration wasn't hurt all that badly, and fuel economy picked up considerably. I think one guy said he was getting in the low 30's (as compared to the 25 that my 123 gets). I figure it's worth a shot and it'll make the cab quieter as well since the engine won't be spinning as fast...

If it works like an domestic (US) car, the gear on the trans end of the cable (driven) can be changed to calibrate the speedo.  The gear in the trans is the drive gear and is fixed to the output shaft.  The speedometer does not need to be changed unless it has gone out of calibration which would lead to a nonlinear display rather than a fixed error.

That's the setup I'm used to...I'm just trying to find out if M-B did this, or what they did. I know that the 126+ electronic speedos are calibrated to the car, so you have to either get it reprogrammed or replace the speedo at the same time the rear end's replaced, with a speedo out of the same type...