The gear ratio refers to the number of drive shaft revolutions for one revolution of the propeller. For example, if an outboard has a 2:1 gear ratio, it means that for one revolution of the propeller, the engine drive shaft turns twice. A lower-unit gear ratio of 1.50:1 means that it takes 1.5 revolutions of the engine to turn the propeller in one complete rotation.
The pitch of a propeller measures the forward movement of the propeller's blade during one complete revolution. A lower pitch prop will increase acceleration and thrust, but top speed will suffer. A higher pitch prop will deliver greater top speeds, but slower acceleration.
The formula for calculating prop performance is (RPM x Ratio) x Pitch = inches per minute. This chart has done the math for us and provides 28 different prop pitch & gear combos to join the 100 mph club.