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-   -   Looking for proper Speedometer /Odo replacement (http://www.peachparts.com/shopforum/diesel-discussion/228725-looking-proper-speedometer-odo-replacement.html)

koldei 07-26-2008 12:53 AM

Looking for proper Speedometer /Odo replacement
 
The odometer on my recently purchased 1985 300TD-t only shows about 80% of the actual mileage.

I know that it is *not* the original speedo.

it currently has a 150MPH speed o that shows ~ 58mph at 3000rpm
( if my calculations are correct, when i show 58, I'm doing 72..unless I'm cracked)
a 'spare' 100mph speedo, along with the 85 mph one from my 1982 240D both show ~ 54mph at the same RPM.

I've found the parts numbers for proper replacement speedo's
(per EPC online seems 85mph and 125mph versions are available)

*o.k. to the point!*

teh parts numbers i have are:
a 009 542 67 06 ( 85 mph)
a 010 542 75 06 (125mph)
a 011 542 68 06 (125 mph)
(along with some others that "are replaced by" newer part numbers such as those above)

the parts numbers that i *SEE* on the speedos are written as:
123.542.37.01 (in the case of the currently installed one)

how do i cross-reference these? (is it as simple as ignoring the leading set?)

I'd hate to spend money on another unit designed for a different rear-end-ratio.

thanks!!

Robert

charmalu 07-26-2008 01:25 AM

I don`t know about your part #. the spedo on my 85 300D has a part#
of 123 542 27 57. my understanding is all 85`s have the 2:88 differential.

60mph Iam@ 3000rpm
65mph 3200 - 3300rpm
55mph 2500 - 2700rpm

the spedo is matched to the diff gear ratio.

hope this helps

charlie

DeliveryValve 07-26-2008 09:23 AM

Quote:

Originally Posted by charmalu (Post 1921302)
..... 123 542 27 57.....

I second that is the correct speedo part # for your '85.

koldei 07-26-2008 12:09 PM

hmm those RPM values got me thinking

*I* show
60mph @ ~ 3100
67mph @ ~ 3500

(on 195/70R14 tires)

those seem to be about in-line then.

leaving me to wonder about my odometer. I took a number of readings vs. mile markers on an interstate over 400 miles round trip.

the ODO seemed to be consistently 80% off
as opposed to working 80% of the time, then stopping altogether for a bit etc
(this is the part there i *assumed* that the whole works was)

maybe another roadtrip is in order.

koldei 07-26-2008 12:31 PM

scary math time
(aka me thinking out loud)

when my speedometer reads 60mph I'mshowing 3100rpm on the tach

3100 RPM * 60 = 186,000RP-hour (engine)
if final transmission ratio is 1:1
186,000RP-hour (driveshaft) / 2.88 =
64583.3 RP-hour rear axle

195/70R14 tires get either 852 or 836 revs per mile dep. on brand

64583/852 = 75.8 miles in that same hour

for better or worse the above seems to work out to what i *thought* was happening..when taking real- world measurements.
(my odo would show 60 miles...80% of the 75miles the interstate said i'd traveled

i am assuming the tach is right


brain = ow

robert


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