Though the 100 mpg car sounds like a myth, it turns out that such vehicles do exist -- only they're built in your neighbor's garage, not a giant production plant. Known as plug-in hybrid-electric vehicles ... they're basically Priuses or similar hybrids that have been equipped with extra batteries, so that they rarely use their gasoline engines at all. People are salivating for plug-ins," says Bradley Berman, editor of the site HybridCars.com. A hybrid vehicle today like a Prius has both a gasoline engine and a battery, which is fed by the braking energy produced by the car. It can't be plugged in. A plug-in hybrid keeps those components, but essentially gets an extra fuel tank, in the form of an added battery bank ... that allows the car to run exclusively off battery power for most driving. Felix Kramer, founder of the California Cars Initiative, a nonprofit group that promotes the use of high-efficiency, low-emission cars, owns the first consumer plug-in in North America. Not surprisingly, he loves it. "Many days I use no gasoline, because I go at neighborhood speeds for under 30 miles, and I'm just all-electric all day," he says. And the mileage? "At highway speeds, you can easily get over 100 mpg." Other plug-in owners offer up similar results. "I used to fill up every 400 miles or so," he says ... "and now I fill up every 800 miles or so." Advocates estimate that it costs less than $1 per gallon to replenish a plug-in hybrid. "Our goal is to have a $3,000 kit," CalCars' Kramer says. (That number, coincidentally, is also what many plug-in evangelists think that the technology would cost for Toyota to add to its hybrids.)