Schneider National will replace about one-third of its tractor fleet by the end of this year, the company said. The national truckload carrier is purchasing 3,000 2012 and 2013 tractors this year, each installed with the OnGuard collision mitigation system. The company has approximately 10,000 tractors.  The vehicles will primarily be Freightliner models, Schneider said, and will be installed at the rate of 75 per week.