Segmentation and Flexibility in Fuel Economy Standards for Tractor-Trailers

White Paper


Therese Langer and Siddiq Khan


The variety of specifications and duty cycles for heavy-duty trucks poses a challenge to the design of fuel efficiency standards for these vehicles. Even within the relatively homogeneous group of tractor-trailers with van-type trailers, there are diverse functional requirements that affect fuel consumption. This paper explores possible approaches to accommodating this diversity in the design of the standards.

Tractor-trailers duty cycles are not uniform. While most tractor-trailers are purchased for long-haul operation, over 10 percent of new tractor-trailers typically travel 100 miles or fewer daily. These trucks have half the annual mileage of long-haul trucks on average and are likely to spend a high percentage of time in stop-and-go traffic, which has major implications for fuel economy. Due to these factors, it is unlikely that any uniform fuel economy standard for tractor-trailers, based on a fixed test cycle, would be equitable. One approach to duty cycle variation would be to classify new trucks based on cab type, given that sleeper cabs are required only for those vehicles that travel long distances. Cab type is not, however, a reliable basis for classification for fuel economy purposes, because trucks having primary trip length of 100 to 200 miles tend to have day cabs, even though their driving patterns are more similar to those of long-haul trucks.

Performance requirements such as the need to pull heavy loads or the need to perform adequately in mountainous terrain are a second reason for the variation in fuel economy across tractor-trailers. While there is no fixed relationship between average fuel consumption and engine rated power, a small engine at appropriate load will tend to consume somewhat less fuel than larger engine will under that same load, due to greater friction losses in the larger engine and a loss of efficiency due to operating at part load. In tractor-trailer engines today, fuel consumption might be expected to vary by over 10% due to horsepower differences alone. Thus a uniform miles-per-gallon standard at fixed load is likely to be substantially more difficult for a truck with higher horsepower. While this may be appropriate to the extent that higher horsepower is used for more aggressive driving, it also could interfere with proper truck specification. In addition, a uniform standard would require that all tractor-trailers be tested at the same load, which could lead manufacturers to optimize fuel economy at the test load, rather than at actual operating loads.

Allowing manufacturers to average fuel economies across their vehicle production would mitigate the problems associated with applying a uniform standard across tractor-trailers with varying duty-cycle and performance requirements. A second approach would be to segment tractor-trailers using one or more parameters that differentiate among them in ways that relate both to fuel economy and to a business necessity. Segmenting by rated power or rated payload would allow testing at payloads reflecting trucks' typical operating weights.

Defining truck standards using a metric such as gallons per ton-mile or gallons per cube-mile may be preferable to a mile-per-gallon standard, given the freight-hauling function of the vehicles. A gallon-per-ton-mile standard, for example, would promote weight reduction by allowing trucks to improve performance by increasing payload while keeping fuel consumption constant. It would not, however, eliminate the difficulties associated with a uniform standard for line-haul tractor-trailers.

The conclusions and recommendations of the paper are:

The variation in fuel economy of tractor-trailers with van-type trailers due to duty cycle and performance-related specifications poses an obstacle to setting a uniform fuel economy standard that is both appropriately stringent and sensitive to the diversity of tractor-trailers. Allowing manufacturers to average fuel economy across their products mitigates the problem to a degree, but functional differences among manufacturers' products may be large enough to raise equity concerns.

o Up-to-date data on the properties and use of new trucks should be gathered to determine whether variations in products across manufacturers are large enough to preclude a uniform tractor-trailer standard, even when averaging is allowed.

Although short-haul tractor-trailers are a small percentage of the new tractor-trailer market, applying the same standard to them as to regional- and long-haul tractor-trailers could make compliance substantially more difficult and costly for some manufacturers. While distinguishing short-haul trucks from regional-haul trucks by physical attributes may not be practical, manufacturers could perhaps be relied upon to classify their own trucks, by virtue of the efficiency technologies they choose to employ to meet the fuel economy standards.

o Separate test cycles should be developed for line-haul vehicles and short-haul vehicles, and regulators should consider allowing manufacturers to choose the test cycle on which a given truck would be certified.

Variations in performance requirements for tractor-trailers, and their effects on fuel economy, also appear sufficient to warrant further segmentation.

o Tractor-trailers should be separated into at least two segments by performance-related criteria, e.g., above and below 400 HP or above and below 60,000 lbs. gross vehicle weight, with a fixed test weight for each segment.

The regulatory challenges associated with the functional diversity of tractor-trailers could also be addressed through an attribute-based standard, rather than by discrete segmentation.

o Regulators should consider defining a miles-per-gallon or gallons-per-ton-mile standard as a function of gross vehicle weight rating, where vehicles are tested at a fixed percentage of rated weight.

Failure to address the variation in tractor-trailer fuel economy, at either the individual truck or the manufacturer average level, could compromise the stringency of the standard. It is therefore important to pursue alternatives to a uniform standard.