trip is right but speedo is 30% fast? - PeachParts Mercedes-Benz Forum

kafer65 · #1 04-09-2007, 10:22 AM

I know the speedo assembly is a replacement. Once I discoverd that the speedo was way off (fast) I assumed the odometer was off too. It hit me that my mileage calculations were off and I was getting something like 19-20 mpg

Well, I checked the odometer Friday and it was dead on. Whats up with that? On the upside with the odometer being correct I get 28mpg now

I average almost 400 miles on a 14 gallon fillup.

#2 04-09-2007, 10:48 AM

If the odometer is correct, you probably do have the correct speedometer installed. Is it a 125 mph? For some reason the (replaced with 125 mph) speedo in my 82 suddenly did the same think a couple of years ago, the odometer stayed correct but the speed started reading significantly high. I disassembled it and did not see any obvious problems. I simply repositioned the needle (trial and error) until it was accurate (within 1-2 mph) at 80 mph using a stop watch and mile markers. It's been fine for about 2 years now, but I still don't know what happened. I know I didn't do a proper calibration, but I wasn't sure it would work at all. If you get to the bottom of this, please let me know.

pdrayton · #3 04-09-2007, 11:26 AM

I am sure my speedo is off too, but the trip is right. Speedo seems to be non-linear, that's the best way I can think of to explain it. If you find out anything, I'd love to know.

Knightrider966 · #4 04-09-2007, 11:30 AM

Quote:

Originally Posted by pdrayton

I am sure my speedo is off too, but the trip is right. Speedo seems to be non-linear, that's the best way I can think of to explain it. If you find out anything, I'd love to know.

mine is off too. About 2 MPH for every 10 MPH of speed progressively. At 10MPH it reads 12 and at 50 MPH it reads 60, etc. My odometer isn't working at all and I know I'll need a replacement at some point.

kip Foss · #5 04-09-2007, 01:40 PM

To reset your speedo is no big problem. I wrote about this before but here goes again.

Determine how much your speedo is off by either following someone who can tell you exactly how fast they are going, or, as I do, use construction site radar units that have big flashing screens to tell you you are going too fast and that the fines are double in a construction zone. I don't know how accurate these are but I think they are good enough.

Remove your speedo from the dash.

Rotate the needle up to say 60 mph. If the speedo is 15 mph fast (you are actually going 45 mph when it reads 60 mph) then hold the shaft that holds the needle and rotate the needle back to 45 mph.

You can do this by either holding the aluminum cup wheel behind the speedo face (wiggle the needle and you will see the cup move), or by grabbing the shaft under the needle with some thin needle nose pliers. Holding the cup is better because you can't really tell if the shaft is slipping by holding it.

JimZ · #6 04-09-2007, 01:47 PM

My speedo shows me going 7-10mph faster than I'm actually going. I don't know how long it's been like that. My trip meter is correct - I measured it against mile markers. It seems like my odometer moves at the same rate as the trip meter, but if the speedo is running fast, does that mean I have less miles or more miles on the car than I think?

MBeige · #7 04-10-2007, 01:55 AM

Quote:

Originally Posted by kip Foss

To reset your speedo is no big problem. I wrote about this before but here goes again.

Determine how much your speedo is off by either following someone who can tell you exactly how fast they are going, or, as I do, use construction site radar units that have big flashing screens to tell you you are going too fast and that the fines are double in a construction zone. I don't know how accurate these are but I think they are good enough.

Remove your speedo from the dash.

Rotate the needle up to say 60 mph. If the speedo is 15 mph fast (you are actually going 45 mph when it reads 60 mph) then hold the shaft that holds the needle and rotate the needle back to 45 mph.

You can do this by either holding the aluminum cup wheel behind the speedo face (wiggle the needle and you will see the cup move), or by grabbing the shaft under the needle with some thin needle nose pliers. Holding the cup is better because you can't really tell if the shaft is slipping by holding it.

I didn't realize this might be my problem. Mine is consistent, 10mph off all the way to 100mph (as far as I remember). Mine reads 10mph faster than it really goes, but the odometer is perfectly fine (I get 400mi fill ups in between).

Would it matter, if the needle waves back and forth sometimes? I mean does that mean anything else?

Ether · #8 04-09-2007, 11:27 AM

I have the same issue with my speedo.

The odometer is dead nuts on but the speedo reads fast. The error on mine gets worse at higher speeds. When the speedo shows 40 I'm going 45, at 60 I'm actually going 70 and at 70 on the speedo I am travelling at about 83 mph.

I've confirmed this with GPS and stopwatch. I've considered just moving the needle but for now I just do the conversion in my head.

vstech · #9 04-10-2007, 01:32 PM

adding mph numbers to the face will not solve the KM that the odo is reading though!
John

kip Foss · #10 04-10-2007, 03:38 PM

Vstech is right about the odometer km to mph change. For that you need new gearing in the speedo head or in the transmission.

If your speedo is off different amounts at different speeds than you have the wrong gearing and moving the needle will not correct that. If the amt. that you are off stays constant through the speed range then your needle is misaligned.