Originally Posted by gtbarry
Use a task automation program to turn on and off mobile data when you connect/disconnect from wifi if you are worried.
I used Tasker to do this - turn off data when in range of saved wifi, turn data on again when out of range of familiar wifi'. I had it working perfectly - even gave it a 15sec delay before doing anything in case i stepped just outside of range for a second (common with our shitty radios on the gnex).
MAJOR drawback for me was that you can't receive MMS messages when data is off. If I could find a way to alter the Tasker profile to turn data on when receiving MMS (then turning off again), it would be great.
All that said, it doesn't save much battery to have data off while on wifi.