Driving mode selection

You can't make PWR mode default, but you can make ECO mode default. that is, if you leave the ECO mode on when you turn the car off, the next time you start the car it'll still be in ECO mode.

Technically they must be able to make the PWR mode as default if they already do that for ECO mode. I wish Toyota gave the PWR mode button the same treatment and let the driver decide. I now have to press the PWR button everytime I start the car, got used to it already.

As for fuel efficiency, PWR mode DOES NOT automatically burn more gas. It all depends on how you drive, local traffic conditions, weather conditions etc, etc. But I found that the rule of thumb is this: you'll get the best fuel efficiency if you use the brakes as littile as possible. This is true for all modes, or even for all other cars, not just the Prius.

you can step on it to accelerate crisply, but don't keep stepping on it until the last 2 seconds then step hard on the brakes. The more gliding you let the car do the better the FE. Pulse to a higher speed than your desired crusising speed and then let it glide, or set it on cruise control even if it's very short distance before the next red light.