Prius owners ?

When driving on the electric motor the gas engine will stop if the speed is below 30 MPH. Above that speed the gas engine will rotate but not use any gas. The reason, if the gas engine is required it is already spinning so can power up much faster.
The important point is that no fuel is being burn while running under electric power.
Having said that, you will find that both the gas engine and the electric engine often run at the same time - accereration will do that - and of course that uses gas.
Have a look at: http://john1701a.com/ and on that page go to: Prius info. Look at: How it works 1 & 2.
Also see: http://prius.ecrostech.com/original/Understanding/WhatsGoingOnAsIDrive.htm

Yeah, Orf pretty much summed it up. Remember that everything goes through the electric motor. i.e. the engine powers the motor that powers the wheel. There's actually two electric motors (one is 10kW, the other is 50kW) which is why it can charge the battery and motivate the vehicle at the same time (the screen's simplified and shows only one motor)