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June 11, 2007

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. 

Comments

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.

Can you get a implied VSL estimate for hypermilers out of those numbers?

I was trying, but I couldn't readily find risk of accidents as a function of tailgate distance.

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.)

Do it like this, if r is something that seems arbitrarily low, r = .000000000001, what is the VSL?

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.

Of course, if you really want to save on gas... drive less...

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.

=)

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

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