Why is the menu in the container in the first place? If you want the menu to span the full width yet the contents of the container are only 1150px I think it is by definition not right to put the menu in the container. Consider restructuring your document. This is an example, I do not have your full code:
<body>
<div id="page">
<div id="header" style="background:Blue;">
header header header
</div>
<div id="mainmenu" style="background:Green;">
menu menu menu menu
</div>
<div id="container" style="width:1150px;margin:auto;background:Red;">
container container container
</div>
</div>
</body>
And if you want the contents of the header and menu to span no farther than 1150px which I think is what you want then consider this:
<head>
<style type="text/css">
.pagewidth {
width: 1150px;
margin: auto;
}
</style>
</head>
<body>
<div id="page">
<div id="header" style="background:Blue;">
<div class="pagewidth">
header header header
</div>
</div>
<div id="mainmenu" style="background:Green;">
<div class="pagewidth">
menu menu menu menu
</div>
</div>
<div id="container" class="pagewidth" style="background:Red;">
container container container
</div>
</div>
</body>