Does disabling 3G while having WiFi save battery? If I have WiFi enabled and 3G data disabled, does it save any battery relative to having both WiFi and 3G enabled?  With WiFi enabled, I didn't know if 3G enabled would make any difference since it wouldn't be used.
Short answer: yes.

It depends on the currently enabled Wi-Fi sleep mode.  By default, Android disables Wi-Fi when your device goes into a sleep. In this case, 3G is the remaining mode of network communication, and it gets used by Gmail/Gtalk push notifications, etc. Obviously, this network communication takes its toll on your battery time. If you disable 3G you'll save some battery, but your device will be completely offline while in sleep.

However, if your Wi-Fi is "always on", your 3G will never be used, so it doesn't really matter if it is on or off. However, "always on" Wi-Fi is a big battery drain by itself...

Update: Note that sleep modes and sub-system energy consumption differ from phone to phone. Moreover, 3G energy consumption varies greatly, depends on which background services are enabled, proximity of the cellular tower, etc. I haven't seen any "authoritative" documentation on this subject.

I suggest you check your battery stats (in Settings -&gt; Battery) and see how cellular communication affects your battery time overall. Usually, it is a very small percentage, so in general dialing your screen brightness a down a notch will have much greater effect on your battery life.