Mythbusters and Hypermilers--Don't try this at home!
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.
You know, this is why they (the engineers) want to get everyone on drive-by-wire systems. Cars (did BMW build one?) that can use sonar to track a car in front, and brake automatically, CAN get that kind of mileage safely.
BTW, I got 65 mpg (rather than my usual 50 mpg) when I followed a friend in a rented U-Hual up the 5 fwy. I wasn't super close.
Posted by: odograph | June 11, 2007 at 09:39 AM
Can you get a implied VSL estimate for hypermilers out of those numbers?
Posted by: John Whitehead | June 11, 2007 at 09:45 AM
I was trying, but I couldn't readily find risk of accidents as a function of tailgate distance.
Posted by: Tim Haab | June 11, 2007 at 09:47 AM
I'm sure it's totally dependent on when and where. Out on the interstate, when the next off-ramp is 20 miles away, there is significantly less chance that a semi will break suddenly. On the other hand, it would be pretty insane to draft a semi into downtown traffic.
(I was thinking more about my 65 mpg experience - it's possible that a more aerodynamic car (the Prius ranks pretty high) would get a residual effect out at 100+ ft.)
Posted by: odograph | June 11, 2007 at 09:57 AM
Do it like this, if r is something that seems arbitrarily low, r = .000000000001, what is the VSL?
Posted by: John Whitehead | June 11, 2007 at 11:22 AM
Nice number crunching, doesn't sound to me like it's worth it! On the other hand, apparently turning your engine off while you're stopped at lights does save you quite a bit.
Posted by: David | June 12, 2007 at 01:32 AM
Of course, if you really want to save on gas... drive less...
Posted by: David | June 12, 2007 at 01:34 AM
Of course, if you really want to save on gas... drive less...
Nah...i choose to make more money for every gallon of gas i use over time...so that the price of gas will be less of a factor.
=)
Posted by: joshua corning | June 13, 2007 at 06:39 PM
I don't agree. My calculations indicate that it can be safely done, if extreme caution is utilized. See the following for my take on this:
http://hamiltonianfunction.blogspot.com/2007/06/blog-post.html
Posted by: Rob | July 14, 2007 at 10:32 PM