I\'ve heard that most speedometers on factory cars are somewhat inaccurate. They overstate your speed, so it appears you are going faster than you really are. I\'ve heard that on my japanese car, the difference is about 3-4 mph in the 65mph region. The error grows larger the faster you go. Why they do this, I do not know. Perhaps it is to give drivers an extra safety buffer against speed limit enforcement.
Now, one might speculate that domestic automakers do not observe this built-in speedometer error policy. If that\'s true, it might explain what clips is talking about. However, I have no personal experience with that, so I can\'t say for certain that domestic automakers design more accurate speedometers.
Also remember that if you install aftermarket wheels which have a different wheel diameter, that can add a noticable amount of error to the speedometer reading. If you change the final drive gear in your differential, which is a popular modification for some auto enthusiasts, that can have a HUGE impact on your speedo readings. You need to install something like the so-called "yellow box" to correct the speedo back to normal.
