If you go into your COMAND, you can calibrate or correct your GPS speed by:

NAVI -> SYSTEM -> CALIBRATION -> TIRE NEW/USED -> FRONT TIRE SIZE

 

Okay, Red your winging it now....those facts you state are like swiss cheese.

 

Then may I suggest a visit to your local Mercedes Benz dealership and ask for the 2004 SL55 and play with the DVD-based COMAND (****pit Management and Data) and see for yourself.

You will find all correction settings discussed here as far the GPS is concerned in the calibration screen.

There is also a hidden diagnostic/function screen that you can access by pressing 3 buttons simultaneously, which will tell you tons more information than you need about other settings.

 

That may be fine, but not every vehicle (even MB, BMW, Lexus) comes with GPS based NAV units. The take rate on NAV is still like 20% for those luxury marks. Regardless, I suspect the computer is using a more accurate value for vehicle speed, but the speedometer is still incorrect. And, worrying about if the speedometer is off 2mph is quite trivial.

I realize you have more experience with these types of vehicles, but I own a 2002 Lexus without NAV. I also own a handheld GPS, accurate within +/- 1 knot (about .9mph) at steady state. In every vehcile I have used it in including my Lexus and F-150, at an indicated 80mph, the GPS told me 78.2mph for the Lexus and 77.6mph for my last F-150. I got a 2004 Dodge as well and it is off a couple mph too. Like I said, every vehicle I have tested or played with has shown an incorrect speed at highway speeds. A very tiny sampling of vehicles, but seems to jive with every other person/third party that has tested speedometers.

Do you have a handheld or portable GPS that you would verify on whatever MB you are referring to?

 

I use the GARMIN portable GPS devices for testing.

The MB also has a speed indicator on the Navi that's sent by a wheel speed sensor.

And the speedometer signal is from the transmission output shaft.