function init () {
    if(!document.getElementById) return false;
    var kiloField = document.getElementById('kilos'),mileField = document.getElementById('miles');
    kiloField.onchange = function() {kilo2mile(this,mileField);};
}

/* AddIfValid function lifted from
http://help.lockergnome.com/lofiversion/index.php/t33046.html
then extended to include isNaN */
function AddIfValid(field) {
        if ((field.value.length == 0) || (field.value == null) || isNaN(field.value)) {
                return 0;
        } else {
                return eval(field.value);
        }
}

function kilo2mile (kiloField,mileField) {
    var tmpVal = 0;
        tmpVal = parseFloat((AddIfValid(kiloField) * 62.137) /100);
        mileField.value = tmpVal.toFixed(2);
}

window.onload = init;