Speedometer
 

Hi All,

Several years ago, my mechanic rebooted the CV axles on my 1984 300D. Part of the deal was that he had a 1985 differential that was just sitting and I was able to get that put back in my car when he pulled my diff. Right now, it looks like my speedometer is off by a few mph at lower speeds and up to about 10mph at freeway speeds. I am wondering if the speedometer needs to be matched to the 1985 differential. Is it possible to buy a 1985-specific speedometer or match my gearing? It would seem the easiest way would be to get a 1985 speedometer. Does anyone have one for sale or know where I can buy one?

Thanks,

Dkr.

I wonder if this can be done by sending your speedometer into a shop and telling them your final drive ratio. Palo Alto Speedometer (don’t know if they’re still around) used to be able to do stuff like this with the gauges out of the car.

Just did a quick google search, and yes, Palo Alto Speedometer is still alive and well....

Check what ratio is on the diff, then find a matching speedometer. Adjust the mileage while installing. Easy

Yep, that part is a no brainer. But, how do you find a matching speedometer? Is there a separate part number for a 1985 car? When I find used listings, they will just specify it is compatible with a 1981-1985 Mercedes 300D. You can tell the 85mph speedometers are for older cars, but other than that, I wouldn't know how to tell what I am getting.

Dkr.

Your 1984 came with a 3.07 ratio differential.

Most probably you have a 2.88 ratio differential (only available in 1985).
If that is correct, the Mercedes Benz Part Number is A0115421906, still available new on eBay.
125mph/200kph gauge face.
(Best bet for longevity of the part)

Or purchase a used speedometer from one of the following vehicles:

1985, 300TD, OM617.952, 2.88, 1.1 Liter
1985, 300D, OM617.952, 2.88, 1.1 Liter
1985, 300CD, OM617.952, 2.88, 1.1 Liter

I have a speedometer from a 1985 300D. I was going to put the 2.88 in my '82. I do not have that car any longer.

Very cool post… I love reading stuff like this.

I never realized height was a function of width via aspect ratio.

Keep in mind if you try to correct this with different sized tires, you're losing the advantage of the 2.88 gear ratio (lower rpms at highway speeds).

1) Inasmuch as the OP has not stated the ratios involved, all discussion and calculation is speculative and hypothetical.
2) IF a 3.07 ratio was replaced by a 2.88, the driveline, and hence the speedometer drive, will rotate at .938 of the original. The speedometer will read slower than actual road speed. At 75 MPH road speed the speedometer will indicate 70 MPH. At 150 MPH there will be a 10 MPH error; a remarkable W123 indeed!
3) The installation of larger diameter tires will increase the amount of error, not offset it. The driveline will rotate even slower for a given road speed.

Oops
 

Frank, I was tired when I recommended increasing the tire size on Post #6
I agree with point 3) above, that larger tires would compound the speedometer error even further.
The incorrect information has been edited and removed from post #6.
(But it still exists in the quote on Post #8).

You don't need to get a new speedo. Just change the driving gears in your existing speedo.

From an older post of mine.

If you can get worm gear from someone, you can get a plastic spur gear from these guys below. I've ordered from them several times without an issue.

https://www.odometergears.com/products/Mercedes-Benz/Mechanical+Speedometer/117

I put my 84 om617 in my 85 gasser body.
 

I wanted use the differential and the speedometer out of the gas body. There's a wire on the differential which is part of the braking system. The 84 didn't have the same setup.

Haven't had it on the road yet but I suspected this would make the SO's reading accurate.

Best reply. Best to take it out of the car yourself, if possible :)
If you don't mind disassembly, indeed you just need to swap out worm gears.
I did that today with my W124 speedo head, the original one was getting worn.

There could be some binding of the speedometer cable. I think that’s why my 300cd has some speedometer bounce even at speed (all my w123 cars have bounced somewhat starting off at low speed, but smooth out by 25mph… except this one), yet my car is doing actual 71mph when the speedometer says 65.

I’ve played with the (new) cable, and the routing affects the speed readout. There’s just enough friction I guess to cause it to not report well.

I am running 15” bundts but the tire calculator says it’s only 1% off…. Not the 10 I observe.
https://i.imgur.com/BPBuKt5l.jpg