[jQuery] innerWidth issue

[jQuery] innerWidth issue

Hi,
i have the following html code:
<div id="#ARGrid">
 <table class='framework'>
  <thead>
   <tr>
    <td class='.gridtitle'>this is a simple test</td>
   </tr>
  </thead>
 </thead>
</table>
here is my css file:
<blockquote style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;" class="gmail_quote">
#ARGrid
{
    border : 3px solid #aaaaaa;
    padding : 0;
    margin : 0;
    font-family : verdana, tahoma;
    font-size : 10px;
}
.framework
{
    background-color : #f0f0f0;
    border-collapse : collapse;
}
.framework,
.framework thead,
.framework thead tr
{
    margin : 0;
    padding : 0;
}
.gridtitle
{
    float : left;
    display : inline;
    height : 20px;
    background-color : #ffdddd;
}
</blockquote>
i want my <td class='gridtitle'> tag to take the whole width of the <tr> (so of the table).
why do i get wrong result using $(".framework thead tr").innerWidth() ?
i get every time 500 px unfortunatelly based on some test i should get 498px.
when <td> is 500 px, it's going outside my div :-(
any idea ?
thanks a lot,
--
Alain
-----------------------------------------------------------