localStorage is for key : value pairs, so what you'd probably want to do is JSON.stringify the array and store the string in the mycars key and then you can pull it back out and JSON.parse it. For example,
var mycars = new Array();
mycars[0] = "Saab";
mycars[1] = "Volvo";
mycars[2] = "BMW";
localStorage["mycars"] = JSON.stringify(mycars);
var cars = JSON.parse(localStorage["mycars"]);