18:1 ratio gives you a finer degree of control.
A great set of machine heads or tuners make an enormous difference in how accurately your guitar gets in tune and stays in tune. That's why Epiphone uses 18:1 ratio machine heads on most of our guitars.
The higher the ratio, the more times you have to turn the tuner for the post to revolve completely. The higher number indicates how many times you have to turn the machine head for the string post to make one full revolution.
Since a higher gear ratio rotates in smaller increments, an 18:1 ratio gives you a finer degree of control when tuning your guitar. Lower ratio machine heads make it harder to reach a precise string tension, which can make it difficult to precisely tune your instrument and can cause the string to slip out of tune.