Speed off in used Mustang, display

Tire size would be irrelevant when standing still. Sounds like the pointer is turned on the speedometer shaft. You can try holding the shaft with a needle nose pliers, then rotate the pointer a little. There is normally a mechanical stop that the spinning metal disc hits to set the position when stopped and to keep the pointer from going the wrong way when you're backing up. If you carefully turn the pointer clockwise past the maximum reading, that stop will be reached again, it will prevent the disc from turning further, then you can force the pointer a little more so it rotates on the shaft. Do that either or both ways until it reads "0" when you're standing still, then carefully press the pointer onto the shaft a little tighter. If it spins too freely, you can glue it in place with a small dot of fingernail polish on the backside of the pointer. You can also glue it with a very small dab of silicone gasket sealer applied with a toothpick, but if you ever have to move the pointer again, it will be hard to get that stuff off. Fingernail polish will crack off easily, then you can reapply it later.

Caradiodoc

You're welcome.

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.

Caradiodoc

How did they make the gear turn only one way? Did something lift it up when in reverse or was there some type of over-running clutch? Seems like either of those would complicate it unnecessarily.

I never changed a gear in a Ford but I used to do a lot of them in my old Chryslers.

Caradiodoc

Yup. All my experience was with the older Chrysler stuff. I have Challengers and Volares. On those it was real easy to pop the pointers off. I really shouldn't suggest grabbing the shaft with a needle nose pliers. They're too easy to bend if you're not careful.

Caradiodoc