Speedometer and odometer are inaccurate - Mercedes Benz SLK Forum

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#1 Old 10-17-2013
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Speedometer and odometer are inaccurate

On my slk 230 the speedometer indicates a higher reading than shown on my gps. The odometer on the other hand indicates less mileage than the gps.

More specifically, the speedometer shows 80 mph when the gps shows 75mph. This same ratio applies throughout the speeds. At 40 it is showing 42 and at 50 it is showing 53 and at 60 it is showing 64. Which works out to about 7% high. However the odometer shows 25.1 miles traveled when the gps shows 26.9 miles. This is just shy of 7% low. The gps is accurate within 1-2 mph and the odo is right on with all my other cars, so I am going to assume that it is accurate. These tests were done on straight roads in Florida so the gps readings should match the indicated odo readings.

I could perhaps understand it better if they were both off the same, either high or low, but one is high and the other is low. And I am using the stock tire size on both front and back.

Anyone else experience this? And, the bigger question is -- What do I do to fix it?
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#2 Old 10-17-2013
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No idea why the mileage reading is out, but nearly all manufacturers speedo's read high, genreally by around 5% - pretty sure there is a reason why but I can't remember at the mo...

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#3 Old 10-17-2013
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Quote:
Originally Posted by Seanhump View Post
- pretty sure there is a reason why but I can't remember at the mo...

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Trying to save your licence!! On my BM you can solder a link between two pads on the display PCB to remove the factory 5% inaccuracy. Once made, the speedo reads the same as the GPS when riding on the flat (GPS speed is not accurate when changing elevation).






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#4 Old 10-17-2013
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Go out on the highway and find some mile posts...then go by at 60 (ie 1 min between the posts). Are they OEM wheels/tires?


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#5 Old 10-17-2013
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That was my first thought, too...the speedometer/odometer are calibrated to the tire diameter, so if it's different than stock the reading would be incorrect.

Can you borrow another GPS to see if the readings there are consistent? I've always found my on-board and external GPS units agree with the speedometer.
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#6 Old 10-17-2013
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Yes, the tires are standard OEM sizes. That was the first thing I checked. What I don't understand is one is reading high and the other is reading low.
I'll try another GPS and see if that makes a difference, but according to traffic around me, my GPS is accurate in that I was keeping up with the flow at 5mph over the limit which is what most folks around here drive.
This will probably be an unanswered question from what I can see. On the other hand, it is kind of cool to see the speedometer at 80 and knowing I won't get a ticket
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#7 Old 10-17-2013
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Our 98 R170 230 is the first car I have ever owned that has a totally accurate speedo. We have owned 30 +/- cars.

It is perfectly in sync with two different GPS units at all speeds.
Stock wheels and stock tire sizes.

I am amazed!
Anyone else out there have an accurate speedo?
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#8 Old 10-18-2013
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You may just be banging up against the manufacturing tollerances .. the Speedo is reading high and is at the top limit of the manufacturing tollerance, and the Odo reads low, again at the lower limit of the tollerances ...possibly!

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#9 Old 10-18-2013
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As I recall reading somewhere that most cars speedo are set low because if there is ever a ticket and the person can prove the speedo is lower than the actual speed (you are going faster than the speedo reads) there are massive fines for the manufacturer. That is assuming that the speedo has not been modified.
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#10 Old 10-18-2013
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Ive always found van and truck speedo's to be spot on accurate but ever car ive owned the speedo reads lower than true speed. My Volvo S80 is way out at a indicated 80 on the speedo the speed by GPS is 73mph and if you go higher it gets worse.

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#11 Old 10-19-2013
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I took the car out on the interstate today and used the mile markers for reference (thank you der Flieger). The odometer is very close, only gaining less than a quarter of a tenth in 18 miles. So that is ok, the gps doesn't do mileage very well, apparently. So now it's just a speed error and I guess I'll have to live with that. Thanks for all the input, lady & gentlemen. I certainly appreciated it.
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#12 Old 10-19-2013
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Clipped this from a Car and Driver article:

"In the U.S., manufacturers voluntarily follow the standard set by the Society of Automotive Engineers, J1226, which is pretty lax. To begin with, manufacturers are afforded the latitude to aim for within plus-or-minus two percent of absolute accuracy or to introduce bias to read high on a sliding scale of from minus-one to plus-three percent at low speeds to zero to plus-four percent above 55 mph. And those percentages are not of actual speed but rather a percentage of the total speed range indicated on the dial. So the four-percent allowable range on an 85-mph speedometer is 3.4 mph, and the acceptable range on a 150-mph speedometer is 6.0 mph."

The EU speedometer spec is here:

http://www.unece.org/fileadmin/DAM/t...gs/r039r1e.pdf
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#13 Old 10-19-2013
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Speedo and odo are independent variables. In the UK it is typical for th speedo to be optimistic, ie indicate a higher speed than actual speed. In other countries, US for example, the speedo may be more accurate. Odometer is usually reasonably accurate. Trip computer average speed is often pretty accurate too, though instant speed is typically aligned with that displayed by the speedo.

GPS systems vary too, but not by much.

A man with one watch knows the time, a man with two watches does not.
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#14 Old 10-19-2013
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Jimin....did you try the distance/time speed check?


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#15 Old 10-19-2013
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No, I didn't. To go 60 on I-75 in FL is suicide. Traffic is usually going 75 to 80+. I'll have to calculate the time from mile marker to mile marker for 75 and then go try it. I did the mileage and it was nearly dead accurate. I'll go back out for the time and get back to the forum.
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#16 Old 10-19-2013
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Jim, the only way to fix this problem is to upgrade to a R171.


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#17 Old 10-20-2013
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You need to get an 09 55!
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#18 Old 10-20-2013
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Jimin... it's 48 seconds for 75 and 45 seconds at 80.


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#19 Old 10-20-2013
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To JB & Mensra- nah, I just bought this one. Boy oh boy, you guys just love to spend my money don't you!!
Seriously, i am very happy with the 230 and it is yellow which matches my other car. The trike isn't mine. A friend shipped it down to have here for the winter and I'm just storing it until he gets here. I had to get a picture of all three.
I just reviewed my post--I should have mowed the lawn before I took the picture !!
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#20 Old 10-21-2013
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mowing lawn or SLKing... no brainer


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