Setting different width and height for gravatar

I’ve been trying to set a different width and height using the get_avatar function. I need to set the author’s avatar in single.php to 60×40 size.

So let’s say the gravatar looks like this:

when set to 60×40, it would look like this (resized and cropped):

However, the default get_avatar does not seem to allow different values for width and height, since

<?php echo get_avatar( $comment, '60' ); ?>

would simply result in 60×60-sized gravatar.

I’m not sure if this is a nice way for doing this, but I tried adding this to functions.php, by facilitating the TimThumb image resizer (I renamed the timthumb.php to display.php):

add_filter('get_avatar','change_avatar_url');
function change_avatar_url($urel) {
    $urel = str_replace("src="", "src="". bloginfo( 'template_directory' ) ."/script/display.php?src=", $urel);
    $urel = str_replace("' class", "&w=60&h=40&zc=1' class", $urel);
    return $urel;
}

but it (seem obviously) does not work.

Is there any way to achieve this?

Unfortunately at moment Gravatar service itself only accepts single number for size and only serves square images.

So you have to achieve it with CSS or download, modify and cache images.