Obviously, knowing your speed is important for legal reasons. "My gauge is missing" is an excuse that's going to cause more problems. If you want to avoid expensive fines or driving record penalties, you're going to need a working speedometer.

A speedometer is also a necessary piece of equipment for Mustangs that aren't road-legal, though. How can you keep an eye on your track improvements if you can't accurately measure them? That's why a high-quality speedometer is a must for any Mustang build.

If you're just worried that your speedometer isn't giving you an accurate reading, then you may need a speedometer gear. If you've made large modifications, like changing your rear gear ratio, then you may need a new speedometer gear instead.