Tires pressure monitoring systems, mandatory on all vehicles under 10,000 pounds but an extra-charge option for heavier trucks, are seeing mixed success attracting light and medium commercial buyers despite promises of longer tread life and reduced tire maintenance. Some fleets say they are unable to justify the additional cost of installing TPMS on heavier vehicles while others say the systems have improved tire performance. NHTSA believes that low tire pressure causes 250,000 accidents per year, killing 660 drivers and injuring 33,000.