Cosgrove estimates that his most recent modifications improve the Firefly hatchback's coefficient of drag from 0.34 to 0.23. While the aerodynamic improvement is an estimate, the fuel mileage benefits are very real and have been calculated at 64 miles per gallon – a 15.1 percent improvement at 56 miles per hour (90 kmh).
So, it's not exactly the most practical way to improve your fuel mileage – access to the rear hatch is, shall we say, limited, and the extra 4.5 feet at the aft end surely makes low-speed maneuvering a bit more difficult – but it does go to show how important aerodynamics are to overall fuel efficiency. Click here for a full description of the modifications and click past the break for a video.
[Source: Ecomodder.com via Wired]