Axle ratio is important to drivability and efficiency. When we say "3.00:1" ratio, that means three driveshaft revolutions for every one axle revolution. This determines where your engine is rpm-wise when you're in straight drive and overdrive. When you have overdrive, you need a higher axle ratio to keep your engine in its power band during cruise. So 3.55:1 and 3.70:1 make more sense with overdrive. In straight drive, you need a lower ratio around 2.80:1 to 3.00:1 to keep cruise rpm conservative.