Of the subprime vehicle loans bundled into securities, 73 percent now exceed five years, up from 64 percent during the first three months of 2014, according to data from Citigroup Inc. Loans as long as seven years are increasingly being put into more bonds as auto-finance companies and Wall Street banks sell the securities at the fastest pace since 2007. While the securities are attracting plenty of buyers with high loss buffers and AAA ratings, some investors are beginning to question the wisdom of lending at terms that have never before extended beyond five years.