Navistar International Corp. (NAV) shares popped 6.7% during premarket trading after Volkswagen AG's commercial-vehicles business said it is considering increasing its stake in the U.S. truck company to more than 17%. The move which would force the company to make an offer for all of Navistar. Volkswagen's commercial truck unit already owns a stake just shy of 17%. Navistar was valued at about $3.66 billion as of last week's close. Before the premarket gains, Navistar shares had fallen 14% to $37.06 so far this year while the S&P 500 has dropped 0.7%.