The Moonbeam is a 100 MPG car that you can build yourself for just $2,500 and 1000 hours. The inventor, Jory Squibb, has provided detailed instructions on his website.
"In a world where the price of oil is trending towards infinity, large companies are quickly seeking alternative energy sources for transportation. Jory Squibb decided he'd build his own fuel-miser, and set about creating it from second-hand motorcycle parts. The resultant vehicle, christened MOONBEAM, gets 80-85 mpg around town and under economy run conditions (max 40 mph) delivers 105 mpg. Two scooters for the bits cost US$500, and the entire project took 1000 hours (a year of 20 hour weeks) to complete, with a total of US$2000 costs beyond the initial scooters. Moonbeam has a Variable speed transmission, so there's no gear shifting (hand controls only)." - Giz Magazine