W123 Speedometer Error - PeachParts Mercedes-Benz Forum

bmwpowere36m3 · #1 01-05-2012, 11:54 AM

Well, my car '81 300CD, has been on a "long" journey of restoration.

617.912 engine, AT, 3.46 diff (original, blown engine)
617.912 engine, 4spd MT, 3.46 diff (european engine)
617.952 engine, 4spd MT, 3.46 diff
617.952 engine, 4spd MT, 2.47 diff (W126 diff)
617.952 engine, 4spd MT, 3.07 diff

Throughout all the changes the cluster has remained original, until I put in the 3.07 diff which I got from a 300TD along with its cluster. I recently checked it vs. a GPS and here are the results.

I'm curious if others are seeing this kind of error as well with their speedometers...

Biodiesel300TD · #2 01-05-2012, 12:01 PM

Your tire size will make a huge difference in speed. When I got new tires on my 240, it calls for a size that is no longer made, I asked the guy if the new tires were going to throw off the speedo. He said no. He was wrong. When the needle is at 55 I'm going closer to 62 or 63. My tires are a little too big. But it give me a little more speed from my 240

.

In your case slightly larger tires might correct the error.

bmwpowere36m3 · #3 01-05-2012, 01:01 PM

I do have 205/70/14 snows on right now, but that would only account for ~2% error.

charmalu · #4 01-05-2012, 01:03 PM

On our 240 we have 195-70-14 tires, and the spedo is the propper one for the differential. we drove 283.1 miles as per the spedo, and the GPS showed 288.1 miles getting to our destination.

Then after a week running around and the trip back home.
spedo showed 804.0 and the GPS showed 790.9 for the total trip.

Charlie

vstech · #5 01-05-2012, 03:35 PM

you must have the speedo from an 85 in your car... the 85 came with the 2.88, and that combined with your 205 tires, could give you the difference...

vstech · #6 01-05-2012, 03:36 PM

no, not even the 85 speedo would account for it... perhaps you have the speedo from a 280CE? what's the max speed indicated on the dial?

Stevo · #7 01-05-2012, 05:26 PM

I started running 195/75/14s and that took care of most of my speedo error in both my 85 TD and 85 240D, at 70, 75 they still read a couple mphs fast, which is close enough for me.

Yak · #8 01-05-2012, 10:23 PM

Wouldn't startek have the speedo applications based on either year or transmission?

bmwpowere36m3 · #9 01-06-2012, 05:17 AM

Quote:

Originally Posted by charmalu

On our 240 we have 195-70-14 tires, and the spedo is the propper one for the differential. we drove 283.1 miles as per the spedo, and the GPS showed 288.1 miles getting to our destination.

Then after a week running around and the trip back home.
spedo showed 804.0 and the GPS showed 790.9 for the total trip.

Charlie

Interestingly enough the odometer reads correctly, at speeds under 50mph there was a 0.1 mile difference every 10 miles or so. However at speeds above 50 mph the difference was greater.

Quote:

Originally Posted by vstech

you must have the speedo from an 85 in your car... the 85 came with the 2.88, and that combined with your 205 tires, could give you the difference...

Nope, came out of a '82 300TD and even the odometer has "82" stamps on in it.

bmwpowere36m3 · #10 01-06-2012, 05:24 AM

Quote:

Originally Posted by Yak

Wouldn't startek have the speedo applications based on either year or transmission?

Not in this case since the car is an '81 300CD, has a 617 Turbo engine, 240D 4-spd MT and 3.07 diff. None of which were ever on this particular model year or in combination with a 4-spd MT.

MBeige · #11 01-06-2012, 11:36 AM

Quote:

Originally Posted by bmwpowere36m3

Not in this case since the car is an '81 300CD, has a 617 Turbo engine, 240D 4-spd MT and 3.07 diff. None of which were ever on this particular model year or in combination with a 4-spd MT.

Some 81 300CD's were turbocharged, but it was a very limited number.

Find the correct speedometer for your drivetrain application (turbo 3.0L model from '82 to '84 should all be the same). Your current instrumentation, which has not been replaced, is geared for a non turbo which will account for the discrepancy.

But you say your speedo came from an '82 300TD. How can that be if your original post states it has not been replaced?

Yak · #12 01-06-2012, 08:50 PM

Something doesn't make sense with the numbers. You shouldn't have a consistent mph difference across a 40 mph speed change - assuming you're in 4th gear the whole time. I would expect the absolute error to change with speed but maintain a consistent percentage difference. Going from a 3.46 to a 3.07, I'd think about 12%. Non-OE size tires will change that.

If you're trying to figure out if there's a speedo that you can use that will get you closer, you'll probably need to figure the different ratios between tire sizes + the tranny + the diff.

This site appears to have the gear ratios and final drives: W123-300D-Getriebe

You're using a TD rear end @ 3.07, but maybe 300D 4 speed gearing @ 3.9, 2.3, 1.41, 1.0?

bmwpowere36m3 · #13 01-07-2012, 06:12 AM

Quote:

Originally Posted by MBeige

Some 81 300CD's were turbocharged, but it was a very limited number.

Find the correct speedometer for your drivetrain application (turbo 3.0L model from '82 to '84 should all be the same). Your current instrumentation, which has not been replaced, is geared for a non turbo which will account for the discrepancy.

But you say your speedo came from an '82 300TD. How can that be if your original post states it has not been replaced?

I guess I wasn't clear, the last configuration has a diff and cluster from a '82 300TD.

Quote:

Originally Posted by Yak

Something doesn't make sense with the numbers. You shouldn't have a consistent mph difference across a 40 mph speed change - assuming you're in 4th gear the whole time. I would expect the absolute error to change with speed but maintain a consistent percentage difference. Going from a 3.46 to a 3.07, I'd think about 12%. Non-OE size tires will change that.

If you're trying to figure out if there's a speedo that you can use that will get you closer, you'll probably need to figure the different ratios between tire sizes + the tranny + the diff.

This site appears to have the gear ratios and final drives: W123-300D-Getriebe

You're using a TD rear end @ 3.07, but maybe 300D 4 speed gearing @ 3.9, 2.3, 1.41, 1.0?

The speeds I checked were in different gears (from 2nd to 4th). I too would expect to see the percentage error be the same across all speeds and not the actual speed. I pulled the 4-spd out of a 240D... so I doubt it was a 300D tranny.

vstech · #14 01-07-2012, 09:16 AM

You can ignore the transmission in relation to the speedo. all you need to match is the diff ratio to the speedometer.
so, find a car that came with the diff you have, and get a speedo that matches IT.

bmwpowere36m3 · #15 01-07-2012, 09:34 AM

Quote:

Originally Posted by vstech

You can ignore the transmission in relation to the speedo. all you need to match is the diff ratio to the speedometer.
so, find a car that came with the diff you have, and get a speedo that matches IT.

I'll have a look at the speedo P/N, but it "should" be the correct one (out of the same donor car as the diff).