Media queries PX vs EM vs REM

Can anyone explain why my media queries break when converting them FROM px TO ems

In the body of my document, I have included the following rule font-size: 62.5%, Therefore I would assume that I could convert my media query to FROM 650px to 65em? Changing my media queries to ems breaks the layout

As an alternative, can I convert the media query to REMS with a pixel fallback ?? that said, I have no idea how to do this

@media screen and (min-width: 650px) { 

body 
{
font-size: 62%;
background-color: #364759;
background-repeat: repeat-x;
background: url("bg.gif");
}

#wrapper, 
footer
{
width: 80%;
max-width: 1050px;
margin: 0 auto;
}
} // end media query 

many thanks, P