correction factors are contrasted with the findings on these issues from other data or engineering analyses.

HDDV fuel economy data are reviewed, with emphasis on average fuel economy derived from surveys. Data on the change of fuel economy with speed derived from simulation models or on-road tests are presented. These data and their relationship to the conversion factor used to convert emissions expressed in units of work to the more familiar units of grams per mile are explored.

Finally, the findings are summarized in the context of the National Research Council's project goals of estimating the effects of expansions of highway capacity.


Truck Classification

The term “heavy-duty vehicles” as defined by EPA covers trucks and buses ranging in weight from 8,501 to 80,000 lb GVW. The GVW is the total weight of vehicle with its maximum payload and is sometimes referred to as gross combination weight (GCW) for truck and trailer combinations. Trucks exceeding 80,000 lb GVW are not allowed on the Interstate highway system, although individual states permit their operation on highways. Their use in off-highway applications such as mining or logging is common. This appendix addresses only those vehicles certified for on-highway use between 14,000 and 80,000 lb GVW, largely because the 8,500- to 14,000-lb GVW class includes vehicles more similar to light-duty trucks.

These trucks and buses are also classified more commonly on the basis of a system used by industry that divides the fleet into eight classes. Classes I and II cover the 0- to 10,000-lb GVW range and include all the light-duty pickup, van, and utility vehicles that are used for personal transportation as well as in light commercial applications. Class III covers the 10,000- to 14,000-lb range and incorporates the “heavy” version of pickup trucks and vans that are used in delivery service or as motor homes. This class also incorporates some imported delivery trucks manufactured by companies such as Iveco and Isuzu (Iveco no longer sells trucks in the United States).

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