Speedometer Calibration?
 

I pass two local police department "safety reminder" speed display units on my way to/from work each day. If the day is rainy (Subaru day), my speedo agrees with their display of my speed. On sunny (Boxster) days, my speedo indicates 3 MPH higher than their units. I guess I could just be happy that I have a 158 MPH top speed Boxster instead of the 'normal' 155 :D , but...

Does anyone know if it is possible to have the dealership easily check/change the speedometer's calibration?

It is possible..
 

A friend of mine had his 98 Boxster into the shop for just that, speedo calibration. Aftwards he claims it is dead on. I don't really mind if it reads a little high, that way when I think I'm doing over the speed limit I am actually not and have a safety margin. :)

I have no idea what it cost him to have this done if anything, but I seem to recall that he said it wasn't hard for the dealer to do.