The axle ratio determines how many times it takes the drive gear to turn the driven gear. A ratio of 3.55 means that it takes the drive gear 3.55 full revolutions in order to turn the wheels one full revolution.

That being said, the higher the axle ratio, the easier it is for your engine to turn the wheels, and the higher the engine will rev to achieve a certain speed. A higher axle ratio will allow your engine to take on a bigger load and accelerate quicker. A lower axle ratio won't accelerate as fast, but will allow for faster speeds per gear (depending on just how powerful your engine is). If you are going to tow a heavy load, in most cases a higher axle ratio will prove more efficient (3.92). If you are going to tow/haul a low to moderate amount of weight, the 3.55 axle ratio will do fine.

While an axle ratio such as 3.55 offers better fuel economy initially, it will suddenly become inefficient when under a lot of stress. A vehicle with an axle ratio such as 3.92 will have worse fuel efficiency when not loaded down, but will not suffer as much as the 3.55 when under stress.

When you have the option of two or more axle ratios, the right choice just depends on your needs.

I hope this helps clarify.