What would cause a speedometer to read higher that the speed I am actualy going in a 2002 cadillac deville? I also have the dash set on mph and not kmh.

Did you change your tire size?

No, it has the factory tires on it. When I hooked up the scanner the scanner woub be accurate but the car would read 40 to 50 mph higher.

The odds are pretty high that, your post not withstanding, your dash is set on metric rather than English units.

................and you don't need a scanner. It's all built into your car. Read the sticky thread at the top of the Forum entitled "How to pull codes", link and scroll down to the link for "...........OBD-II".