How can a driveline be spaced to get the most fuel economy?
Fuel economy potential is maximized with a heavy-duty engine by specing the driveline to operate the engine in the most fuel-efficient speed range at the desired vehicle cruising speed. This is commonly called the “gear fast/run slow” concept. This driveline spec can yield substantial gains in fuel economy, but it has a few inherent drawbacks that should be understood: Potential vehicle speed is increased. This extra speed must not be utilized if optimum fuel economy is to be realized. Rated engine speeds above 1800 rpm are not recommended for applications with limited driver control. Rated speeds of 1600 rpm generally provide acceptable control of vehicle speed with this driveline spec. Vehicle response at cruising conditions is reduced due to reduced overall gear reduction. There is less engine speed loss on grades before a downshift is required. Thus, a “gear fast/run slow” spec could require more downshifting than a “gear slow/run slow” spec.
