function calculate_mpg_savings(gasform) {
  var dailymiles = gasform.dailymiles.value;
  var costpergallon = gasform.costpergallon.value;
  var daysperfillup = gasform.daysperfillup.value;
  var sizeoftank = gasform.gastanksize.value;
  var mpg = Math.round(((daysperfillup/sizeoftank) * (dailymiles)) *100)/100;
  var monthlycost = Math.round((1/mpg) * costpergallon * (dailymiles * 30.417) * 100)/100;
 
  document.gasform.mpg.value = mpg;
  document.gasform.monthlycost.value = "$" + monthlycost;
  document.gasform.yearlycost.value = "$" + Math.round(monthlycost * 12 * 100)/100;
}