Speedo & tach reading incorrectly - Page 2 - PeachParts Mercedes-Benz Forum

Winston Wolfe · #16 02-09-2014, 10:28 PM

I was already thinking it would be kinda nice to have the 2.88 rear...maybe that's what I should be doing instead of messing with the cluster? I think some of the big SDL/SLs of the era had those diffs, right?

vstech · #17 02-09-2014, 10:45 PM

only car that had the 2.88 is the 1985 123-300D or the 126-300SD

all other years had different ratio. later models had 2.65 or higher...

Winston Wolfe · #18 02-09-2014, 11:04 PM

Thanks for clarifying!
I'll work on getting the cluster pulled to verify the year/equipment.

Skippy · #19 02-09-2014, 11:11 PM

Taller tires cover the same ground in fewer revolutions. That causes a speedometer to read low.

205/70 14 comes out to a nominal diameter of 646.8 mm by my math.

195/70 14 (the stock size) comes out to 632.8 mm.

The difference in tire diameter alone would cause an otherwise correct speedo to read 2.2% low. It's part of your inaccuracy, but not the whole story. I'm curious to see what you find with the gauge cluster.

Edit: I've been driving around with a similar combination to the OP for a bit over 20,000 miles. The 3.07 rear is great around town, but a little buzzy on the highway. A 2.88 might be a nice change. However, I think ideal would be to keep the 3.07 and find a five speed (I know...not many of them on our side of the pond).

Winston Wolfe · #20 02-09-2014, 11:29 PM

Honestly, the 4-speed manual on a turbo in a coupe was too cool to pass up.

And I don't MIND the 3.07, however if putting in a 2.88 is going to fix my speedometer issue, I'm happy to do that.

charmalu · #21 02-10-2014, 12:30 AM

What is the part number at the bottom of your spedo cluster on the face?

A 2:88 Differential will have a spedo with the part# of 123-542-28-57

Charlie

Winston Wolfe · #22 02-10-2014, 08:51 AM

Oh...well...that was simple!
The number is 123 542 18 57

vstech · #23 02-10-2014, 10:33 AM

Quote:

Originally Posted by Winston Wolfe

Oh...well...that was simple!
The number is 123 542 18 57

heh, I get the boobie prize for first correct diagnosis!

vstech · #24 02-10-2014, 10:35 AM

I've got a buddy in altoona, he might be persuaded to bring you an 85 diff... (I have a few...)

Winston Wolfe · #25 02-10-2014, 10:49 AM

I'm confused...
I thought that 123 542 28 57 was the 2.88 diff.
Mine is 123 542 18 57.

vstech · #26 02-10-2014, 11:30 AM

Speedometer part #

vstech · #27 02-10-2014, 11:32 AM

it's not really the part number for the speedo... it's a stock number from VDO... mostly irrelevant to the year the speedo came from...

physically check the number stamped on the diff case. verify it's a 3.07
if it is, you have a speedometer from an 85 looking for input from a 2.88 diff...

Winston Wolfe · #28 02-10-2014, 11:43 AM

I'm sorry; I wrote that badly. What I meant to say is that I thought that 123 542 28 57 was the cluster number that should correspond with a 2.88 diff--and that mine was 123 542 18 57 instead.

I'll try to get under the rear with a creeper at lunchtime (tons of salt and puddles in the lot right now!).

Winston Wolfe · #29 02-10-2014, 12:35 PM

I saw the stamping; it said 307.

vstech · #30 02-10-2014, 12:58 PM

ok. so now all you need is a speedometer from an 82-84 W123 to stick in there, or you can search for an 85 diff to match... but since you have a manual trans, you better stick with the 3.07 I think you may be burning the clutch too much with a 2.88.
there are members on here with the 2.88 behind a manual, but I'd want a bit shorter a gear...