light-duty vehicles, which results in an aerodynamic power consumption 4.7 to 7.7 times higher. The benefits of aerodynamic features are a strong function of both operating speed and annual vehicle miles traveled (VMT). As Figure 5-7 shows, aerodynamic drag is larger than rolling resistance at speeds above 48 mph for a typical current truck. At 32 mph, however, aerodynamic drag is only half of tire rolling resistance, and aerodynamic drag becomes insignificant at low speeds. The sensitivity to VMT applies to any fuel-saving feature: the more miles a vehicle travels, the larger the potential fuel savings becomes.
In determining whether to apply aerodynamic features to a vehicle, it may be appropriate to consider a duty cycle average road speed hurdle. A method to quantify a weighted aerodynamic-average speed (WAAS) has been established that provides for an average of the mileage-weighted velocity3 (V3). If it is deemed that a speed hurdle is appropriate, a numerical value for the hurdle speed must be established, and the WAAS must be verifiable. For example, will a tractor-container/trailer chassis operate at a low average mph by virtue of its operation over short distances between ports and rail terminals? CARB has taken this issue into account in its greenhouse gas (GHG) regulation, where drayage tractors are exempt if operated within 100 miles of the port. If this approach is applied in the general case, it would likely require use of electronic onboard data recorders to substantiate the short distance and/or below-speed-hurdle reality. The required record keeping and oversight could become very burdensome.
There are four regions of the tractor-van trailer combination truck that are amenable to aerodynamic design improvements. These regions include the various tractor-related details, the tractor-trailer gap, the trailer skirt, and the trailer