>I phoned the service department for our other car and they told me what shows >on the speedometer should be the actual speed.

Thats odd. No measuement device is 100% accurate and most cars are
off by a bit (if nothing else, tire wear will affect it as the tire
diameter decreases). I would think they would know that. I bet a
speedometer shop would not say this (and might be a good source to
ask about acceptable tolerance since they spealize in this area).

10% does seem a bit too "loose", though. I thought dealers could
adjust it based on tire size since I have seen reports of people having
that done when they change to a slightly different tire size.