Speed/RPM figures for w123 - PeachParts Mercedes-Benz Forum

JHZR2 · #1 04-29-2010, 12:39 AM

Hello,

I know there are a few threads like this, but here I go.

82 300CD. Always have said the idle is low, sits just off the 500rpm point.

Verified that my speedometer is accurate via GPS. It is in fact spot on to the GPS value.

Driving along, at 60 MPH, I get 2100, and at 70 I get 2600 RPM.

Both seem low. Not sure if there is a consistent drop in rpm reading for some reason.

I swapped the tach amp with a known good one. This information I have is consistent, all the time.

Any ideas?

Lycoming-8 · #2 04-29-2010, 01:17 AM

The car is close to 28 years old, so unless you were the original owner, you can't be sure that 2.88 gears from an '85 weren't swapped in at some time. Our '83 runs about 2800 RPM at ~68 mph, and it appears to have the stock 3.07:1 gears.

okyoureabeast · #3 04-29-2010, 05:01 AM

Those numbers seem consistant with my 85's 2.88 differential.

There's a way to figure out the gearing by some numbers on the differential. A search can pull it up.

Sorry I can't do it, I'm at the airport right now.

JHZR2 · #4 04-29-2010, 08:49 AM

Yeah I am sure the ratios are indicated on the diff, and that could be used, along with AT ratio and tire size to give me an indication.

vstech · #5 04-29-2010, 09:06 AM

you can also check the part # on the speedometer, and compare it to an 85. I bet you have 2.88's in there.

JHZR2 · #6 04-29-2010, 09:14 AM

Interesting. Speedo is spot-on, so if there was an '85 diff installed, the speedo was too.

That said, does anyone know the transmision gear ratios???

I am assuming that 4th gear is 1:1?

Reason I ask is because Im using a tire/diff/transmission gear ratio calculator to try to get an idea of RPM versus speed. Im using this one:
http://www.kabamus.com/garage/gears.html

So using 195/70-14 tires and 1:1 for transmission ratio, I cannot get the right RPMs for the two points Im using, 60 and 70 MPH, which were both correct per my GPS.

If I put in 2.88, I get 51 MPH at 2000 and 64 MPH at 2500 RPM, both are off compared to my readings.

If I put in 2.47, I get 59 MPH at 2000 RPM, which is close to my noted 2100 RPM for 60 MPH on the road, but at 2500 RPM it is saying I should be doing 74, whereas I know that at 2600 Im only doing 70.

So the error Id like to remove from this is transmission gear ratio in 4th gear. Any idea?

Also, my tach still seems to read low. I have to wonder if there is some inherent error in my tach reporting the right RPMs. Ive changed the amp, and the tach is smooth and consistent, could there be a way for it to have a consistent error in its reading???

Thanks!

tfernandez · #7 04-29-2010, 09:23 AM

I got 3000 RPMs this morin going 70 on the high way in my 84 coupe. donno if that helps at all

JHZR2 · #8 04-29-2010, 10:16 AM

That's about what I recall for my 83 sedan...

toomany MBZ · #9 04-29-2010, 11:04 AM

The tach reads off the front crank, so that's right regardless of the diff or speedo.

JHZR2 · #10 04-29-2010, 11:30 AM

Understood, unless there is some sort of gearing off the signal in the tach (could my tach be from another car?), or some sort of issue in the electronics of taking the signal to the amp, to the tach...

vstech · #11 04-29-2010, 01:06 PM

a 2.47 ratio installed, would lower your rpms, but increase the slippage from the TC, so your reading of 2600 at 70 sounds right.
I am now thinking you have a 2.47

samboyellowsub · #12 04-29-2010, 01:07 PM

the speedometers generally read about 5-7 percent high anyway. What happened was the 2.88 differential was put in which compensated for that small discrepancy in your speedometer. after all, 2.88 is 93.8 percent of 3.07 so there you have it - a difference of about 6.2 percent.

toomany MBZ · #13 04-29-2010, 02:41 PM

Quote:

Originally Posted by JHZR2

Understood, unless there is some sort of gearing off the signal in the tach (could my tach be from another car?), or some sort of issue in the electronics of taking the signal to the amp, to the tach...

It's an electronic signal, not sure if you could "gear" it.
http://en.wikipedia.org/wiki/Hall_effect_sensor

DeliveryValve · #14 04-29-2010, 04:56 PM

You'll need to check the differential ratio before getting the right answer.

But I am leaning towards your tachometer being faulty.

The other question I have is what is the part number on the face of your speedometer?
For your '82, I believe the correct part number would be 123 542 85 01 and that would be an 85 MPH speedo calibrated for a 3.07.

Alberta Luthier · #15 04-29-2010, 08:49 PM

I would really like to know what part number your speedo is. I have a 247 in my 300CD and am VERY happy with it but my speedo is now off by about 24%!!!