Updated here to satisfy John's pestering...
I had some spare time last night and decided to catch up on my backlog of DVR'd TV shows--as I've claimed before, DVR/TIVO is THE single greatest invention of the past 20 years. One of my favorites, Mythbusters, were tackling big rig myths--particularly does drafting (driving close) behind a semi improve gas mileage as claimed by hypermilers? The results:
- 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.
The recommended safe distance to drive behind another vehicle traveling 55 mph is 150 feet. The risk of something bad happening--like dieing--increases the closer you are to the vehicle in front. On a 100 mile trip at 55 mph in a car that regularly gets 25 mpg, you will save $2.07 if you drive the entire distance 100 feet (1.25 seconds) behind a semi. You will save $3.86 if you drive 10 feet behind the semi (.124 seconds). Oh, and you will be labeled an a$$h@!& driver.