49 CFR Part 571 121 Amends the FMVSS on air brake systems to improve the stopping distance performance of truck tractors. The rule requires the vast majority of new heavy truck tractors to achieve a 30 percent reduction in stopping distance compared to currently required levels. For these heavy truck tractors (approximately 99 percent of the fleet), the amended standard requires those vehicles to stop in not more than 250 feet when loaded to their gross vehicle weight rating (GVWR) and tested at a speed of 60 miles per hour (mph). For a small number of very heavy severe service tractors, the stopping distance requirement will be 310 feet under these same conditions. In addition, this final rule requires that all heavy truck tractors must stop within 235 feet when loaded to their “lightly loaded vehicle weight” (LLVW).