// outer width plugin 

$.fn.sumOuterWidth = function(useMargins) {
  var sum = 0;
  this.each(function() { 
    sum += $(this).outerWidth(useMargins);
  });
  return sum;
};

// example sum the width of the first 2 li's:
//$('ul li').slice(0,2).sumOuterWidth(true) 

// Also gets the sum of the width of the first 2 li's:
//$('ul li:eq(1)').prevAll().andSelf().sumOuterWidth(true);