This will only work if you log in I think. Why not simply create /etc/rc2.d/S00SetupSpeed and put the same line in there ?
/etc/rc.local is executed in runlevels 2 thru 5 by /etc/rc*.d/s99rc.local, which is a link to /etc/init.d/rc.local
Your method would break the intent of /etc/rc*.d, and would likely be overwritten upon upgrading.
The scripts in this directory are executed each time the system enters
The scripts are all symbolic links whose targets are located in
To disable a service in this runlevel, rename its script in this directory
so that the new name begins with a 'K' and a two-digit number, where the
number is the difference between the two-digit number following the 'S'
in its current name, and 100. To re-enable the service, rename the script
back to its original name beginning with 'S'.
For a more information see /etc/init.d/README.