I installed coastaltech.com's ev "switch" successfully, as have many other Prius owners. I don't know if it works on the HiHy. Instead of an actual button, you install this switch inline to a wire harness behind the glovebox, and when you pull the cruise control lever for 2+ seconds, the Prius beeps. You're then in EV mode until you exceed 33mph, accelerate hard, the battery goes too low, or you pull the cc lever again to deactivate EV mode. If the battery pack is too hot, it won't allow you to trigger this.

However, I've had mixed results with it. Drive too long with it turned on and then the engine kicks on because it has to charge up your batteries. That kills the mileage you just saved.

Where I have success with it is when I pull off the freeway en route to work. It's about a half-mile to 3/4 mile to my work's parking garage from the offramp, so I activate EV mode and cruise into work at ~30mph. It doesn't use too much battery, and if it does, the next time I turn on the car the engine has to kick on anyway.

On a side note, energysavingman, my fiancee and I did the same thing you and your wife did. Highlander 2wd Hybrid and Prius, except I'm the Prius driver 95% of the time. I hope you add your mpg info to the database!!

Rick