Warning: Don't try this! The safest distance to drive behind a big rig at 55 miles per hour is 150 feet. Driving any closer is insane because it puts you in the driver's blind spot and also does not give you enough time to respond if the big rig's driver changes speed. This post is for informational purposes only.
Drafting is a technique, usually used by race cars, to reduce drag by driving closely behind another car. Mythbusters did an episode about Big Rig myths that included a look at the drafting... myth? Does anyone really consider drafting a myth? Anyway, they found drafting increases your miles per gallon up to 39 percent at 10 feet distance away from a big rig. At two feet apart however, they found miles per gallon actually went down.
The first part of debunking the drafting myth took the Mythbusters team to small wind tunnel to prove putting things in front of other things blocks wind. Good thing, too. I know I've always questioned stories of objects blocking the wind and can only be convinced by a wind tunnel experiment. Sarcasm aside, while the episode was dangerous and 39 percent is kinda shocking, Mythbusters should not have done this!
No one thinks drafting is a myth and the information they gathered only encourage people to try this. What's next for Mythbusters; how much will over-inflating your tires increase your mileage? How much fuel can you save by turning off your engine at high speeds? How many engine parts can you remove and still drive your car? Mythbusters did warn that driving so close to a big rig is insane but doing tests that find the drafting sweet spot looks like encouragement to me.