How can I limit data usage depending on whether the screen is on, or off/locked? How can I limit data usage depending on whether the screen is on, or off/locked? For example, when the screen is off, data can only be sent and received at a reduced rate. Or, when the screen is off, data connectivity is reduced to 2G. I want to leave data on, but I don't want to let it chew through all of it.

I am aware of the data saver option, but it doesn't do what I want.
Use Tasker app which can disable the internet connection automatically when the screen is off. Configure the app as below:

Profile: Screen Off

Context: (Event) Screen off

Task:

Clear variable %SCREENSTATE
Stop if %POWER ~ ON
Disable "Mobile Data" (With "Keep MMS" if you want)
Disable "Auto-SYNC"


Set Profile's Task Priority to 6