Before choosing a gear, let us consider why to change your Mustang's gear ratio. First and foremost, differential gears do not change power levels (you will not see an increase in engine HP or torque). Rather, they manipulate what power and torque (mostly torque) is available to you at a certain RPM, to help you either accelerate faster or have a higher top speed. By stepping down to a lower gear (a numerically higher ratio, i.e 3:1 is lower than 2:1), effectively you are raising engine RPM per wheel revolution. What does this mean? In a nutshell, a lower gear ratio (remember, a lower gear actually has a higher numerical value!) will help you step off the line faster by manipulating torque. Essentially, you hit the power band sweet spot in less time. The trade off? Well, because the engine is now turning a higher RPM for a lower overall speed, you of course are going to sacrifice top end speed because you’ll simply run out of RPM (this is also known as “running out of gear”).
I like to use a mountain analogy to explain this. Think of a high gear (lower numerical ratio) as a very tall, steep mountain, with the peak representing top speed. When you swap over to a lower gear (numerically higher ratio), imagine taking the peak off of the high gear mountain and adding it to the base of the low gear mountain.
Interpreting these ‘mountain’ diagrams, we can see with the high gear that top speed is greater, engine RPM at a given speed is lower, and time given to reach a speed is higher. Examining the low gear diagram, we see that overall top speed has been lopped off, engine RPM has increased per any given speed and subsequently time to reach a given speed is less. Now, with that out of the way, let’s move on to actually choosing a gear!