Speedometer way off
 

got the car pretty well done changed an 82 240D to an 82 300DT, changed the engine, trans, rear end, and speedometer everything came out of the same 300D but when driving the speedometer is close to 10 MPH off I didnt check the rear end ratio before I put it in but if I need to Ill get it up in the air and look at the tag, but the speedometer number is 123 542 63 01. I think thats the right speedometer for an 82 300D turbo, the tires are not the correct size but should not make a 10 mph difference if anyone has any ideas as to why its so far off it would help allot

what size are they? 10mph is not unbelievable. I think this is about what mine is. How are you verifying the speed offset?

the tires are 195/70R14's, I checked the speed the best I could without a radar gun or a GPS, I took an average after driving behind friends with the cruise set to dead on 60

for information purposes........
 

not exactly relevent here. but for info......

http://www.miata.net/garage/tirecalcold.html

this be for those who are lazy.

it's pretty possible for the odo to be from any of the other 123 vehicles out there, they are so simple to change out. what year was the donor car?

240 is a 82 and 300 was an 82, didnt know if you could tell what that speedometer is supposed to fit by the part number off the face

what MPH does the speedometer go to?

I think 85 I would have to check to 100% but thats what I remember

85 is the correct answer. so the speedo is right, it must be the tires, or the diff. time to crawl under there and find out what diff you have. my bet is that it's a 2.88 from an 85 model.
is it off over or under the indicated?

hard to say what had been done to the car that I got the parts from it was in a junk yard, your going faster then the speedometer says

yup, classic 2.88 dif symptoms... time to find an 85 speedo... good luck, there aren't very many out there.
on the plus side, you will have a 125MPH speedo then.

well at least I know whats going on, so does anyone had a 3.07 rear diff, or a speedometer for an 85 with a 2.88

Hold on. I dont think its a 2.88 diff. A 3.07 speedo should be 110mph or so. and 85mph speedo is indicative of a ~3.48 rear.

not true.
the 3.07 speedo from an 82 turbo 300D would be 85MPH as per the stupid fed rules.

the only way to accurately check your speedo that I have used is by driving an indicated 60 on the interstate and time a mile. One mile equals one minute at 60 mph. If it is less than one minute you are going faster than indiacted, and vice versa.

Tom W

what car would a 3.48 rear be from? a 240D? a manual tranny car? all the turbo's from MB had the 3.07 rear except for the 85, which had the 2.88, and all vehicles built in 82 had the 85MPH speedo regardless of the tranny or rear ratio.

My 84wagon has a 110mph speedo, did feds change rules between 82 and 84?

He needs to look at the diff and make sure its a 3.07 (it will be stamped on it iirc).

late 81, to early 83 all had 85MPH speedo's thank you very much jimmy carter...
and thank you very very much Mr. Reagan for fixing it.

well looked at the number on the back of the diff and its a 3.07, turned the drive shaft and watched the tires to make sure some one hadnt changed out the gears and its for sure a 3.07 so Im back to stage 1 what in the world could be making it that far off ? the tires might do a couple mph at the most so im kinda at a loss

Remember tires are a sliding scale, the faster you go the farther offset they are.

the 300 must have had a 240 speedo in it,... either that, or you mixed them up during your swap... as the 240 had anywhere from a 3.46 to a 3.69 rear depending on tranny.

unless some one changed the speedo in the junk yard car before I got it I know it came from the 300D and it has a tach that the 240 didnt have also. Anyone with an 82 300D want to check there part number on the speedo face and see if it matches mine ?

better to look at the K factor on the rear of the instrument. VDO calibrates them to a factor that MB uses, its the "Konstant" or K factor.
I never wrote down the K factors over the years I've messed with VDO speedometers but that is the key thing a speedo shop uses. Not the faceplate p/n.
Are you aware that speedometers also age as the magnets lose some of their pull against the rotating drum inside the speedometer? Swapping in a used one may still leave you with an error as most old speedometers are off
If you like the speedometer and its got clean odometer wheels, I'd just carry it to a speedometer shop that knows what they are doing and let them cal it. Or leave the car off and they will nail it down to within 2% (as close as you can read the pointer)
BUT if you insist on using the faceplate# here's a thread on this subj.
http://www.peachparts.com/shopforum/1589564-post19.html
I note that 123-542-03-67 isn't listed.
Also I suggest you use the search feature on this matter as there is lots of info already available

I think that a speedo shop is probably the best thing for me to do, I hate to change the speedometer again this one only has 60K on it and is in great shape. Thank you for all your help on the subject it was driving me up the wall trying to figure out what was going on

The 300d non turbo came with the 3:46 gear. Maybe someone put in a speedo from the 300d non turbo not knowing the difference. Did you check against the mile markers?

The hash marks on the speedo for maximum engine speed will tell the tale of which rear end they are for.

204ds all came with the 3:69 rear except the limos which were never brought here, or the cars equipped with the 15" wheels and tall suspension for extra ground clearance which came with the 3:92.

Tom W