Understanding Gear Ratios: Different gear ratios change how the power is applied to the air. As the letter gets further away from "A" (more and more gear reduction) the RPM and pitch speed is reduced and the static thrust (and usually prop diameter) is increased. In other words the prop shaft will turn slower but with more torque so it can handle larger props. If the pitch speed does not exceed the stall speed you can't pull the model fast enough to make it fly. You usually want pitch speed to be at least 2-2.5 X stall speed for slow fliers, trainers and 3D models. Sport aerobatic models 2.5-3X stall speed, pattern airplanes at least 3X stall speed. If your experimenting be sure to get a range of props for the motor. In many instances one prop will make the model feel poor while another prop might exceed your expectations. Nobody can predict what will work best for you so get what you think will work and a couple nearby props so your not missing out on better performance. Look in props section for simple math to calculate pitch speed if prop pitch and RPM are known.