You don't change the sensor - just the plastic gear on it. There are many websites with gear calculators, so do some searching. This one is for CrownVics, but it might work for you:
Speedo Gear Selection
It took me a few seconds to figure out that you were talking about a modern car. Falcons haven't been made in the US for some time, so it would help if you add your car's specs to your signature &/or profile.