Toyota Prius
(Toyota Motor Sales, U.S.A., Inc.)

The 2017 Toyota Prius hybrid earns EPA-estimated mileage ratings of 54 mpg in the city and 50 on the highway. One of the first questions buyers ask is “How does it get better mileage in the city than it does on the highway?”

The main reason is a technology called regenerative braking. It converts the motion (kinetic energy) of your car and returns it as electrical power to the vehicle, increasing its efficiency. Regenerative braking (sometimes shortened to regen) is used in all of the hybrid and battery-electric cars currently offered in the U.S., plus a few gasoline-only powered cars.

In a traditional car, a tremendous amount of energy is wasted by the braking system. When you put your foot on the brake, the surface of the brake is pushed against the hub’s spinning disk or drum. Disc and drum brakes do a fine job of stopping the car, but all of the kinetic energy of your car’s momentum is irrecoverably lost to heat from friction.

Electric and hybrid vehicles drive the wheels with an electric motor or multiple motors. When you start to coast, each motor instantly becomes an electrical generator, working to capture the car’s kinetic energy. Operating as generators, they both create a drag on the wheels they're attached to and start producing electricity.

[Read: What Is a Plug-In Hybrid?]

As soon as you depress the brake pedal, the generator is told to increase its power generation, which in turn causes even more resistance, slowing the car. The electricity generated flows back to the battery pack, where it’s available to help accelerate the vehicle, or power its systems.

Regenerative braking systems are most useful when you drive in stop-and-go city traffic, where the power from each stop at a traffic signal helps to launch you when the light turns green. That’s why the EPA-estimated mileage of hybrids is generally higher in the city than it is when you drive on the highway, where you are not decelerating as often.

If you drive down a long hill, however, you’ll actually see the range display on the car’s dash start to increase as the battery pack is charged.

The more time that a hybrid vehicle can operate on electric power alone or with a boost from its electric motor, the more fuel efficient it will be. It’s least efficient when you drive in an environment that requires its gasoline engine to do most of the work. In pure electric cars, any power put back into the battery pack increases the car’s range. Efficient hybrid drivers learn to control the balance between having the electric motor do the work, and having the gasoline internal combustion engine speed the vehicle along.  

Though the 2017 Mazda3 isn’t an electric car or hybrid, models equipped with the i-ELOOP system use a small generator to collect energy created when the car is slowing. The energy captured powers its auxiliary functions, such as air conditioning, lights, and infotainment system. By taking that work load off of the gasoline engine, Mazda3s equipped with i-ELOOP pick up one mile per gallon of fuel efficiency.

[Read: How Does a Hybrid Car Work?]

Of course, regen braking systems don’t turn your car into a perpetual motion machine. There are mechanical losses in the systems, and the regenerative systems share the braking load in most cars with traditional brakes. Most regen systems will not fully bring the car to a stop – the mechanical brakes take care of the last few miles per hour.

In most electric cars and hybrids, only two wheels are driven by electric motors, so mechanical brakes take care of the other two. Regenerative brakes can only provide a limited amount of braking, so when you brake hard or encounter an emergency, the car’s friction brakes provide much of the stopping power. Of course, the heat energy that the friction brakes create can’t be reclaimed.

In early hybrids, the handoff between the regenerative and friction braking systems wasn’t very smooth. As hybrid and electric vehicle technology evolved and became more refined, the pedal feel got better. In the newest models, such as the 2017 Honda Accord Hybrid, the brakes feel natural with hardly any disruption in feel as the different systems engage.

A great side benefit of regenerative braking systems is that they take significant wear and tear off of the vehicle's mechanical brake systems. In other words, you won't have to change your brake pads as often, decreasing your total cost of ownership.

Though today’s systems rely on computers to create the greatest efficiency while preserving driving comfort, regenerative braking systems are not new technology. They were used on early electric vehicles around 1900, and have been used on trolleys and trains for more than 100 years.