Apparently Toyota omitted placing a EV push button switch on North American Prius 2004+ models. This push button, is normally pre-installed at the factory for Japan and European versions.
Here is a well documented hack that shows Prius users how to install their own EV push button.
It appears that placing the Prius in EV mode could, possibly, improve fuel efficiency. But the owner’s manuals of the European and Japanese Priuses with the EV button warn that use of the EV button could LOWER your MPG.
Make sure you read the full article and understand when to use the EV push button and when not to use the EV push button.
And as always, try not to blow up your Prius when performing this hack, and remember we cannot be held responsible if you do.
For those Prius owners who would like the benefit of the EV push button but who do not want to get their hands dirty, there is an after market solution. See links below:
Prius 2004 EV Push Button
After Market solution