sub fuel tanks

No. Completely independant.

You could actually run out of fuel , even with the Sub tank full.

There is no automatic fuel transfer (other then the small amount at startup). You need to do it.

If the sub tank gauge is moving with the main tank.

Either , gauge is stuffed , or , the pump is running full time , and constantly dumping fuel into the main tank

As Flappa mentioned, there is a small amount of fuel transferred each time you start the vehicle ( assuming the main tank is not full ). If you do a lot of short trips the sub tank will empty fairly quickly.

A mate of mine drives a patrol and told me that if he just drives short trips around town that the sub tank can be more than half empty by the time the main tank fully drains. He reckons a 1/2 litre or so must get dumped from the sub tant to the main tank every time you start the motor.

every time you start the car a small amount of fuel is pumped from sub to main, i think it was something to do with condensation?
so yes, plenty of sharts = sub goes down

Hi , if you take the fuel cap off the main tank and have someone start the car you should be able to see the small pipe that sends fuel from the sub to the main just inside the pipe and see fuel pumping through,this can be seen on most GUs , this is done to prime the transfer pump , it is a good idea to keep some fuel in the sub for this reason .