So wird es klappen
<html>
<head>
<script>
function Auto(){
this.tachostand = 100;
}
function fahren(pkw,weg){
pkw.tachostand = pkw.tachostand + weg;
}
</script>
</head>
<body>
<script language="JavaScript">
var PKW = new Auto();
document.write("Tachostand: " + PKW.tachostand);
fahren(PKW,400);
document.write("Tachostand: " + PKW.tachostand);
</script>
</html>