Best way to accelerate

The efficiency of regenerative braking is just less than 50%. That means that all of the energy that you get from regeneration is at only half the efficiency of using the ICE alone. If you were driving a conventional car with a 1.5 liter miller cycle engine, your mileage would be better than in the Prius, but the accelleration would be unacceptable. The Prius is a hybrid so that a highly efficient small gas engine can have acceptable throttle response. There are small exceptions to this broad generallization, but they are by and large, inconsequential. Regenerative braking in the Prius will increase your mileage by only 1-2 MPG depending on your driving style and conditions. The most efficient style of driving is to almost never use the brake and to accelerate without the electric assist except at very low speeds. The most important thing you can do to increase MPGs is to keep speeds low as at highway speeds, 5 mph will cost you about 5 MPG.