The only stupid question is the one you don't ask.
EV means "Electric (only) Vehicle".

When your Tachometer is below zero RPM, the gas engine is not going at all. There is a little EV icon on there, isn't there?

So when the tach is below zero, and pointing to EV, you are an all electric vehicle then. No gas at all is being used.

This is possible after the car engine is warmed up to normal, you do not need to have the Air Conditioner or defrost running, and you are moving less than 40 MPH. The car goes to EV more easlily when you are using the brake, or going less than 25 MPH, or are totally stopped.

-John