Company tests on a broad range of boats indicated fuel savings of 14 to 54 percent, when running in the auto mode versus fully trimmed down in manual mode. The key is Active Trim's exclusive and patented GPS-based control system. Unlike systems that use only engine rpm to control trimming, Active Trim will employ boat speed and engine rpm. This approach solves problems with the engine trimming up instead of down, if the propeller breaks loose in hard turns. Mercury says. It also solves issues with the engine trimming up too early or too late while the boat is climbing on plane.