Day 4 with the E420 ... speedometer issue ...
 

Day 4, and still loving the new (to me) E420.

Still appreciating the power over my 300E.

But .....

I put my GPS in, and noticed that my car speedometer is optimistic by about 10%, at 100 kmh.

My first thought was, hmmmm, undersized tires?

a lower rear end? (higher numerically)

But then, I zeroed both my trip odometer on the car, and the distance travelled on the GPS.

Now, after having done about 100 kms since zeroing, I find that the car odometer is optimistic, but only by about 1% compared to the GPS. So, that would indicate correct gearing and tire size. In any case, I double checked the tire size and those are correct.

So, any idea?

What causes a speedometer to be 9% different than the odometer?

Actually - GPS VS car speedo
1992 W124 300E 2.6 - no difference
1995 W124 E320 - 10% speedo is cheating
1995 A124 E320 - 5 %

The general idea is psychological - you can tell your friend / significant other how fast you drove, or your significant other can scream on you to slow down before entering "Street racing" zone.

Regards

I thought someone had said that car manufacturers set speedometers to be slightly off. or something like that.

+-3% would be one thing, 9% is a bit much. If it really bugs you (it would me :) ), I'd look into finding a speedo repair shop that specializes in European (VDO) cars & have yours sent off to be (re)calibrated. There are a few places in the US, not sure about Canada.

I thought I heard or read somewhere that it's actually a law (possibly in Europe only) that the speedo reads higher than actual. Not sure by how much. I can try researching that.
Gilly

Yup:
Speedometer - Wikipedia, the free encyclopedia

Speedometer - Wikipedia, the free encyclopedia

Interesting.

I read the article, and I understand the rationale behind mandating that the displayed speed must be greater than actual road speed.

But it raises a further question.

It seems highly unlikely that Mercedes would randomly choose a different design error for each car, although it is possible that the manufacturing process itself introduces random error. In either case, I can't imagine that some 1994 E420s would have 10% error, while others had 1 or 2%.

A reminder that, although the article cites improper tire size as a source of error, my odometer reads within 1% of the GPS, so tire size is not an issue here.

So, have others here compared their GPS speed readings with speedometer readings?

Here is another article with test results of various makes/models speedometer accuracy in kilometers.
All of the Mercedes (albeit all recent models) on the list tested within 5kmh or 3mph at 100kmh.

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German cars in particular seem to have way optimistic speedometers, maybe all the "autobahn" hype is just BS because the pointer is 20 degrees further :D

Meanwhile typical American car is often right on the money...

Well, yeah. It's a "Euro" thing, they have to be optimistic, it's the law. Not here in the US, but the Europeans just do it on all of them i guess. Since they have to have different speedometers in the US cars you think they would make them more accurate than the Euro-spec cars.

I have an E420 and a CL500. There is an informational speed sign on a main road. The speed limit here is 45MPH and the device is a way to slow you down. I have noticed no problems with either of my two cars. They are both dead on at 45MPH (or faster). Same with my recently sold 320CE.
Anziani

There IS a law.

It states that the indicated speed on the speedometer cannot be lower than the actual speed, but it can display a speed that is higher by up to 10%.

It is not the law, the law does not forbid an accurate speedometer, which is not difficult to implement particularly with modern motor vehicle design. The difference in tire radius from wear is trivial as well. There is a big difference of 1% accuracy and 10% plus 4 (imagine... you are seeing 200 km/h on clock and going only 178?!). Even 5% (test above) on a $60 000 car or whatever is unacceptable.