rankmirrors
is included in the pacman-contrib
package.
Make a backup, then run the rankmirrors script through sed to remove some extra commented-out lines.
mv /etc/pacman.d/mirrorlist{,.old}
rankmirrors -n 5 /etc/pacman.d/mirrorlist.old | sed -e '/^##/d' > /etc/pacman.d/mirrorlist