For a certain hardcore segment of the eco-underground, the fuel economy of a standard Prius – about 50 miles per gallon – just isn't good enough. With a relatively simple upgrade of additional battery power, a standard hybrid-electric car can become a plug-in hybrid that gets up to 150 mpg. The California Cars Initiative's Calcars.org has all the info you'd ever want on retrofitting your hybrid.

CCI is a group of eco-geeks of all stripes – from entrepreneurs to engineers – working to build both demand and market receptivity for cars that get over 100 mpg. “In Hybrids, We Can Tackle National Security, Jobs and Global Warming At The Same Time,” goes the CCI motto.

Here's how plug-in models work: Standard hybrids are rigged with an additional battery unit that can be stored in the spare-tire compartment. It can be plugged into any old electric outlet in the garage and recharged overnight. On the morning commute, the car can go up to 30 miles on the extra battery power before it has to rely on gas, and get up to three times the mileage. Beyond the eco-benefits, the savings are impressive: You can get the equivalent of 50-cent-a-gallon gasoline given current electricity prices.

Photo credit: Metrokc.gov