I wonder... are you trying to change the speed on an existing saved game? If so, you have to change it another way.
In your menuconf.tab file in your pakset's config folder, in the section for dialog_tool entries, there will be one for 27 that is likely commented out with a #.
Change it to:
Or change ~ to any other key you want to bind to this command.
Now open back up your save game and press ~ (or whatever key you assigned) and you will get the settings menu. You can change bits per month there. Then, all you need to do is save the file, then reload it and it will start with the changed speed setting.