The federal government requires the U.S. fleet of heavy-duty engines and vehicles—including commercial long-haul tractor-trailers—to meet various requirements, including those for safety, fuel economy, and air pollution emissions. The first federal emissions standards for heavy-duty engines and vehicles were introduced in 1974 and were gradually tightened in a number of steps. On January 18, 2001, the U.S. Environmental Protection Agency (EPA) set current emission standards for criteria air pollutants and their precursors, including nitrogen oxide (NO x ) and particulate matter (PM). Further, on October 25, 2016, EPA and the National Highway Traffic Safety Administration (NHTSA) jointly published the second (current) phase of greenhouse gas (GHG) emissions and fuel efficiency standards for medium- and heavy-duty engines and vehicles (Phase 2). The Phase 2 requirements set emission standards for commercial long-haul tractor-trailers, vocational vehicles, and heavy-duty pickup trucks and vans , and they phase in between model year (MY) 2018 and MY 2027. The rule expands on the Phase 1 standards (promulgated on September 15, 2011, for MYs 2014 through 2018) and introduces first-ever controls on trailers (i.e., the part of the vehicle pulled by the tractor) and glider kits and vehicles (i.e., a new chassis combined with a remanufactured engine). This report examines EPA's and selected other federal agencies' requirements on glider kits and glider vehicles and outlines the congressional response. A glider kit is a tractor chassis with a frame, front axle, interior and exterior cab, and brakes (see Figure 1 ). It becomes a glider vehicle when an engine, transmission, and rear axle are added. Engines are often salvaged from earlier model year vehicles, remanufactured, and installed in glider kits. The final manufacturer of the glider vehicle (i.e., the entity that assembles the parts) is typically a different entity than the original manufacturer of the glider kit. Glider kits and glider vehicles are produced arguably for purposes such as allowing the reuse of relatively new powertrains from damaged vehicles. Four original equipment manufacturers (OEMs) currently produce glider kits in the United States: Peterbilt, Kenworth, Freightliner, and Western Star. Numerous companies of varying sizes unassociated with the OEMs serve as the final manufacturers or assemblers of glider vehicles. These companies specialize in installing remanufactured main components from donor trucks (commonly Detroit, Cummins, and Caterpillar engine options) into new glider kits purchased from the OEMs. Representatives of the glider assembler industry assert that the benefit of a glider vehicle over a new truck is a more reliable and fuel efficient vehicle that requires less maintenance, yields less downtime, and yet offers a range of currently available safety features and amenities. Further, they report that a glider vehicle is approximately 25% less expensive than a new truck at purchase, which makes it popular with small businesses and owner-operators. The Phase 2 standards require, among other provisions, that all glider vehicles be covered by both vehicle and engine certificates. The vehicle certificate requires compliance with the GHG vehicle standards of 40 C.F.R. Part 1037. The engine certificate requires compliance with the GHG engine standards of 40 C.F.R. Part 1036, plus the criteria pollutant (i.e., NO x and PM) standards of 40 C.F.R. Part 86. Under the Phase 2 rule, used or rebuilt engines may be installed in glider vehicles, provided that they meet all standards applicable to the year in which the assembly of the glider vehicle is completed, or they meet all standards applicable to the year in which the engine was originally manufactured and also meet one of the following criteria: the engine is still within its original useful life in terms of both miles and years, the engine has less than 100,000 miles of engine operation, or the engine is less than three years old. Thus, the standards allow for installation of relatively newer engines in glider kits for purposes EPA deemed consistent with their original intended use—the salvaging of relatively new powertrains from vehicle chassis that have been damaged or have otherwise failed prematurely. The Phase 2 rule has a transitional program for glider manufacturers. For calendar year 2017, each manufacturer's combined production of glider kits and glider vehicles is capped at the manufacturer's highest annual production of glider kits and glider vehicles for any year from 2010 to 2014. Any glider kits or glider vehicles produced beyond this allowance are subject to all requirements applicable to new engines and new vehicles for MY 2017. Effective January 1, 2018, the permissible number of glider vehicles that may be produced without meeting the Phase 2 long-term requirements is limited as follows: Small businesses may produce a limited number of glider vehicles without meeting the long-term engine or vehicle requirements (or larger vehicle manufacturers may provide glider kits to these small businesses without the assembled vehicles meeting the long-term vehicle requirements) capped at the small vehicle manufacturer's highest annual production volume in 2010 through 2014 or 300, whichever is less. The 2018 allowances mostly continue after 2020, but effective January 1, 2021, all glider vehicles are required to meet the Phase 2 GHG vehicle standards. EPA cited its authority to regulate glider vehicles as Clean Air Act (CAA), Section 202(a), which authorizes standards for emissions of pollutants from new motor vehicles that cause or contribute to air pollution that may reasonably be anticipated to endanger public health or welfare. Regarding its statutory authority to regulate criteria pollutants under the CAA, EPA noted that it has broad authority to control all pollutant emissions from "any" rebuilt heavy duty engines (including engines beyond their statutory useful life) under CAA Section 202(a)(3)(D): Rebuilding practices.—The Administrator shall study the practice of rebuilding heavy-duty engines and the impact rebuilding has on engine emissions. On the basis of that study and other information available to the Administrator, the Administrator may prescribe requirements to control rebuilding practices, including standards applicable to emissions from any rebuilt heavy-duty engines (whether or not the engine is past its statutory useful life), which in the Administrator's judgment cause, or contribute to, air pollution which may reasonably be anticipated to endanger public health or welfare taking costs into account. Regarding its statutory authority to regulate vehicle-based GHG emissions, the Phase 2 rule defines the completed glider vehicle as a "motor vehicle" under CAA Sections 216(2 and 3): (2) The term "motor vehicle" means any self-propelled vehicle designed for transporting persons or property on a street or highway. (3) Except with respect to vehicles or engines imported or offered for importation, the term "new motor vehicle" means a motor vehicle the equitable or legal title to which has never been transferred to an ultimate purchaser; and the term "new motor vehicle engine" means an engine in a new motor vehicle or a motor vehicle engine the equitable or legal title to which has never been transferred to the ultimate purchaser. Thus, according to the Phase 2 rule, if a used engine is placed in a new glider vehicle, the engine is considered a "new motor vehicle engine" under CAA Section 216(3), because it is being used in a "new motor vehicle." In short, EPA argued that "it is reasonable to require engines placed in newly-assembled vehicles to meet the same standards as all other engines in new motor vehicles." Additionally, CAA Section 202(a)(1) not only authorizes EPA to set standards "applicable to the emission of any air pollutant from any … new motor vehicles" but states further that these standards are applicable whether such vehicles "are designed as complete systems or incorporate devices to prevent or control such pollution." Thus, according to the Phase 2 rule, the CAA not only contemplates but in some instances directs that EPA establish standards for "incomplete vehicles" and vehicle components for purposes of controlling emissions from the completed vehicle. With this interpretation, EPA adopted provisions in the Phase 2 rule stating that a glider kit becomes a vehicle when "it includes a passenger compartment attached to a frame with one or more axles." The Phase 2 rule contains no emission standards for glider kits in isolation, but the standards for glider vehicles reflect the contribution of the glider kit. However, manufacturers of glider kits are subject to production caps as discussed above and would be required to obtain certificates of conformity before shipping any glider kits based on EPA's interpretation that the kits are considered "incomplete motor vehicles." The production and use of glider vehicles has the potential to increase emissions of criteria pollutants—specifically NO x and PM—from heavy-duty vehicles. Current EPA emission standards for NO x and PM (which began in 2007 and took full effect in 2010) are at least 90% lower than previous standards (see Table 1 ). Thus, EPA estimated in the Phase 2 rulemaking that NO x and PM emissions of any glider vehicles using pre-2007 engines could be 10 times higher than emissions from equivalent vehicles being produced with new engines. Additionally, based on prior standards, EPA estimated that NO x and PM emissions of any glider vehicles using pre-2002 engines (i.e., before exhaust aftertreatment requirements) could be 20-40 times higher than those of current engines. These emission impacts are compounded by the recent increase in sales of glider vehicles. Estimates provided to EPA during the Phase 2 rulemaking indicate that production of glider vehicles increased by an order of magnitude since 2006—from a few hundred each year to several thousands.