• the batteries get too discharged (down to 2 bars on the Prius' Energy display)
• the vehicle speed hits 35mph
• the driver asks for a lot of power (climbing a steep hill or getting onto the highway)
• the drive disengages EV Mode

When one of the above conditions is met the gasoline engine will turn on and the Prius will be out of EV Mode and back into its normal driving mode.

Because of reasons no one quite understands the Prius that you buy in the United States is missing the button on the dashboard to initiate EV Mode (but we all know that it's somehow because of lawyers, right?). The computer in the US-market Prius knows what EV mode is and what it's supposed to do when it's in EV Mode -- it's just that there's no way to tell the it that you want it to engage EV Mode. Unless you hack your Prius a bit.]]>
Every Toyota Prius that rolls off of the assembly line has the programming built into the computer to enable “EV” (“electric vehicle”) Mode. EV Mode tells the computer controlling the Prius to turn off the gas engine and use only its batteries until one of the following things happens:

• the batteries get too discharged (down to 2 bars on the Prius’ Energy display)
• the vehicle speed hits 35mph
• the driver asks for a lot of power (climbing a steep hill or getting onto the highway)
• the drive disengages EV Mode

When one of the above conditions is met the gasoline engine will turn on and the Prius will be out of EV Mode and back into its normal driving mode.

Because of reasons no one quite understands the Prius that you buy in the United States is missing the button on the dashboard to initiate EV Mode (but we all know that it’s somehow because of lawyers, right?). The computer in the US-market Prius knows what EV mode is and what it’s supposed to do when it’s in EV Mode — it’s just that there’s no way to tell the it that you want it to engage EV Mode. Unless you hack your Prius a bit.

I bought a kit from Coastal Electronic Technologies that uses the cruise control stalk to tell the Prius that you’d like to initiate EV Mode. Other people have figured out other solutions using normal momentary switches to accomplish the same result. I chose the cruise control solution because it seemed like an easy installation (it was) and because I didn’t want to have to install another switch onto my dashboard and run the wiring for the switch.

The kit came with very detailed instructions complete with plenty of pictures. The installation involves removing the glove compartment and installing a few wire taps into one of the harnesses which run behind there. The taps connect to the cruise control wires and to a spot on a connector where the EV switch would’ve connected if the people at the factory had installed it. Some people have reported problems getting the taps to make contact properly but I didn’t run into any such trouble. My total installation time was about 30 minutes.

Once the kit is installed you’ve joined the rest of the Prius-owning world and can initiate EV Mode by pulling the cruise control stalk towards you for 2 seconds. You’ll stay in EV mode until you disengage it (pull the cruise control stalk for 2 seconds again), until you meet one of the conditions mentioned above, or until you turn the car off.

I have to say that I installed the EV Mode kit because I thought it would be a fun gadget to play with but I find that it really is a useful thing to have. It’s nice to be able to pull the car out of the garage to park it outside or to drive from one store to another without having the gasoline engine start. And, when used in the right situations, it can improve the Prius’ already-fantastic gas mileage.

So, if you’re a Prius owner in the U.S. give EV Mode a shot. Think of it as installing optional standard equipment on your Prius.