Hypermiling is the sport of squeezing a gallon of gas until it screams. One popular technique is drafting, or driving really close behind big rigs to get into the low-pressure zone created as they move through the air. According to Discover online, driving in this "free ride zone" not only save fuel for the tailgating driver, but also for the trucker, who is getting a little high pressure push.

Until something happens. Tim Haab at Environmental Economics shows test results from Mythbusters:

In scaled wind-tunnel tests, driving 100 feet behind a semi at 55 mph will reduce drag on your car by 40%. The drag reduction increases as you approach the bumper of the truck until you get a 93% drag reduction at a distance of 2 feet.

In road tests, the testers achieved an almost 20% improvement in gas mileage at a distance of 100 feet (at 55 mph) and a 45% improvement at 10 feet.

Tim also calculates that at 100 feet you have 1.25 seconds to respond if the truck slams on the brakes, (keep off that cell phone) and at ten feet you have .124 seconds. The reccommended distance at 55 miles per hour is 150 feet.

Conclusions: there are better ways to save fuel. ::Environmental Economics