The theory is that the faster a prop rotates the more power it takes just to turn it in the water. A slower turning prop with more pitch is more efficient than a faster turning prop with less pitch even though the calculation for theoretical speed is the same for both.

The chart on the left shows the prop shaft speed for a given engine rpm/drive ratio. You can use it to see the effect of changing drive ratio, without changing the propeller pitch. The difference between a 1.36 drive and a 1.5 drive is about 10%, or 500 RPM at 5000. If your boat was running 5000 RPM with a 1.5 drive the prop shaft RPM would be 3333. If you were to change to a 1.36, find the same (closest) prop shaft RPM in the 1.36 column and you see that the engine RPM would drop to  4500. As each inch of prop pitch is about a 200 RPM change, you would have to go down 2 1/2" in pitch to pull the same RPM. Our prop calculator will help with this math.

The overall ratio is a combination of both the upper and lower ratios combined. In the case of a 1.5 Bravo the upper is 32/27 (1.185) times the lower at 19/15 (1.266) = 1.50.

The downside to a higher reduction in the upper is that there is more torque on the lower gears, vertical shaft  and prop shaft. In the case of a Bravo there is also more load on the gear floor.