I don't mean to dismiss Ford Man's reply. First a little "theory". When the pointer is always off by 15 mph no matter how fast you're going, the pointer just needs to be repositioned. The clue is yours is reading below 0 mph when standing still. It can't do that because of the mechanical stop, so there's proof it's the pointer.
When the wrong speedometer gear is in the transmission, or you change tire size, your speedometer will always be off by a percentage, not a specific amount. That means it will read 0 mph when you're standing still, 15 mph too fast at 50 mph, (in this example), and 30 mph too fast at 100 mph. The faster you go, the greater the error. That's not what's happening in your car.
That said, a former coworker had a Grand Caravan that always read about 7 mph higher than it should have. It didn't matter if he went 30 mph or 60 mph. It always read 7 mph too much. But it did read 0 mph when standing still. This was a different kind of speedometer than yours though. Instead of a speedometer cable, it used a speed sensor and a computer to run a "stepper" motor which has magnetic coils that pulse the pointer to the desired positions. It has a mechanical stop for the pointer itself, not the armature of the motor. According my story the pointer needed to be repositioned, but the coworker replaced the gear in the transmission and the speed was correct at every speed after that. I wanted to argue with him but you can't argue with success. Later I learned the reason for his success. The scale of the speedometer wasn't linear, meaning the distance from 0 - 30 mph was different than from 30 - 60 mph. So while the percentage of error remained constant, the non-linear scale corrected most of the error. That can't happen to a cable-driven speedometer.
Friday, January 7th, 2011 AT 5:15 AM