all of the "current speed" signs in my area run off of older X band radar technology (what brand/make I couldnt tell you).
keep in mind radar guns have to be calibrated for accuracy, so theres a higher possibility that the SIGN is wrong and not your car.
IIRC the MS3 uses a mechanical speedo gear, which would be really hard to be set innacurately (assuming stock tire hieght...)
chuyler is right though - most cops arent going to pull for more than 10+ mph. Its a waste of time as it could be anything from your car being off to their radar gun being off... unless they are extremely bored/have a clear reading on you. in my state up to 9mph over, its a slap on the wrist.
more than 10mph over the speed limit and its clear to both the officer and yourself that you are speeding. you may not realize a few miles over, etc.