AutoblogGreen

Actually, I can attest to the turning-off the car part at speed. There's a stretch of freeway that is very empty for about 4 miles going downhill when I come home from work. I can almost always turn the car off to the pre-ignition position and coast all the way down.

I usually pick up speed to about 70 mph where wind resistance does not allow any more speed to be picked up.

Over one tank, I usually gain around 50 additional miles. My mileage there goes from 27 MPG to about 31 MPG...well worth it if you ask me.

I do it cautiously however...too much traffic and it's not worth it.

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Realistically, in urban driving, how often can you drive at least 150 feet behind a big rig? I would say that average following distance is more like 50-60 feet and in Detroit traffic 30 ft or sometimes less is necessary lest somebody squeezes between you and the truck. That would be more dangerous than being behind a truck. Trucks brake more slowly so you have more time to react. And when there's lots of traffic you have to believe that truck drivers assume a vehicle is behind them at all times.

Drafting big rigs up close happens all the time. It's not safe, but it's safer than the alternative of being behind a car or SUV that is more likely to make sudden movements. Trucks just can't move as suddenly and their high view of the road allows the drivers to react in advance. They decelerate more slowly, so you're better off. Just don't drive under the truck's trailer and have that bumper bar knock your head off. :-)

UH2L
http://www.thingsivenoticed.com

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I've always liked the Mythbusters, but they seem to be running out of ideas, and the new shows are getting steadily worse. I guess there are only so many interesting myths, and once you've exhausted them all...

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The trick however, is that it does not actually reduce overall milage. And the reason for that is that the rig in front actually has to work harder to pull the car along behind it. The bubble of air behind the rig gets bigger, increasing drag.

It's the first rule of thermodynamics. There ain't no free lunch.

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Sorry Ernie, you are incorrect. What actually happens is that the vehicle behind the first one actually ends up pushing the car in front.

If you've ever watched NASCAR, you'd know that two cars drafting together will run faster down the straights than two cars alone (and no, they aren't physically touching). This would only be possible if drag were reduced on the car in the front.

Yes, the car doing the drafting gets the majority of the benefit, but saying that the car in front works harder to compensate is incorrect.

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I see in the future a special lane where you have to have a GPS and RADAR assisted auto-drive feature that locks your car into a WIFI enabled virtual train. If one car slows down...they all slow down. Just think of the 39% savings for hundreds of cars in a virtual long-haul high way train. It would save space. I see this becoming an opt-in system where cars that have the capability are given access to special designated lanes. This technology exists today it just takes the energy to get it put into practice.

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"Mythbusters should not have done this" Why? Do you think people will now go out and draft big rigs because there is a gas mileage benefit? How about some personal responsibility? If anyone is dumb enougth to try this they should not be driving period.

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@ Throwback

I couldn't agree more. It's not a TV show's responsibility to do anything beyond entertain. Mythbusters also ran a show on shooting bullets into water. As it turns out I can sit 5 feet under water and let me brother pump .357 Magnum rounds at me all day, they'll never get me. Am I going to do this now that Mythbusters proved my safety? NO!

You don't go playing golf in a lightning storm just because the weather channel didn't predict lightning.

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Where I am from (Detroit), as another user mentioned, drafting is an every day occurrence. While I may be driving with a 50 foot buffer (which, in my opinion, for an experienced driver in a car with decent brakes is a safe distance to follow at), which according to many people would be considered drafting, I will very often get cut off by one OR TWO people! Yeah, as in bumpers being only 3-5 feet away from each other. THAT is unsafe. Following behind a tractor trailer, especially if you use a CB radio to tell them what you are doing first, can be a safe practice. I personally don't do it as habitually, but I don't think that it is nearly as unsafe as everyone makes it out to be. Idiots weaving through traffic at 100 mph are the real threat to other drivers.

As for why it works, a simple example comes to mind. Think of a Canadian geese flying V. The ones in the back are getting a FREE (or partially discounted) ride. They take turns though, and semis usually don't get such an offer.

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Yes, drafting isn't safe -it's really just tailgating with a different purpose...but having said that....

Dave is correct....drafting helps the car/truck in front too. This concept has been proven valid for bike racing too (and there is NO violation of laws of thermodynamics involved here).

Drafting works best if you are in an non-aerodynamic car/SUV/truck. When I was in college, drafting really worked with my old boxy Volvo station wagon. However, with my current Prius, it doesn't seem to help much at all. Not worth it, especially factoring in the safety issue.

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if you have the instant MPG display in your car you can see this. moving van trailers work the best because they are closer to the ground in the rear than a standard box trailers which allow the air to pass under.

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Drafting big-rig trucks is extremely safe. Have you ever seen or heard of a big-rig truck stopping quickly? They can't. Here is part of the reason why:
http://www.txinjurylawfirm.com/CM/AdditionalResources/Trucking-Accident-Info.asp

Although air brakes are a safe way to stop large vehicles, it takes one-half second or more for the air to flow through the lines to the brakes which impacts the stopping distance for the vehicle. The air brake lag distance at 55 mph on dry pavement adds about 32 feet to the stopping distance. A truck driver traveling 55 mph under good traction and brake conditions must contend with a total stopping distance of over 300 feet, a distance longer than a football field.

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Chris,

The Interstates I drive (I-81 from Southwest Virginia and part of Western Maryland) are already like your virtual auto-train. Following distances of about 2 car-lengths are typical. I agree with the article that this kind of following distance is insane, but if you try for 150 feet, you'll end up with an SUV and a ricer cutting you off thereby reducing your spacing to the 2-3 carlengths. I typically see the cleanup of a couple of accidents on each 320-mile trip, and it seems that the cops are too busy cleaning up accidents to bother much with speeders, tailgaters, or left-laners.

Anyway, this article has tempted me to go out and purchase a ScanGuage II. I've been thinkning about it for a while, but getting some real-time numbers on what happens when I pop into the passing-lane from behind a vehicle that I'm passing would be interesting.

Anyway, no electronics required -- impatient people on an over-capacity highway will draft despite themselves, since they seem to think that tailgating the last car in a 10-car line will speed people up, regardless of the pileup risk. I'd love to know where you live so that I could move there, and share the road with sane, well-educated, safety-minded drivers! :-)

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I have a car with one of those MPG displays and pay some attention to it. You get "drafting" benefits in many situations, not just driving behind a big rig. For example, being slightly behind and to the side of the truck, or even another car or set of cars. All the same situations that bicyclists use when they race / ride in pelotons.

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To go one step better, I'd like to have a mechanism that attaches my car to the bar that's on the back of big-rig trailers. Then I could coast and read a book. ;)

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Regarding @1: Under the "don't try this at home heading"... Check it out on your car first, but some US cars with slush boxes won't start back up unless that car is in Park. I've stumbled on it one time when my Buick stalled at about 60 mph after pulling onto a freeway on ramp. Turning the key to start did nada. Put the prndl in "N", nada. Not gonna save too much gas that way, but check your car first! That and most cars need the engine running for the power brakes and power steering. Ironically '87 Turbo Regals all have electric power brakes.

That said, I drop my stick shift into neutral and let out the clutch (saves the throw-out bearing among other things) on EVERY downhill that gets me a positive speed increase. Of course we're not talking 4 mile hills in my commute, but it is rolling for about 25 of the 37 miles. There is only one hill where my car will pick up more than 15 MPH on the downhill. That is one thing that helps me get 29-30 MPG per tank on a car with a 26 MPG HWY pre-2008 EPA sticker. Hey I burned the gas to get up the hill, why not get some of that back going down the hill!

I'm not sure you'd see TOO many more miles between idle and shutoff and granted EVERYTHING we're taking about in the post is going to be branded reckless or dangerous by Click-n-Clack and probably by your significant other, at least having brakes and steering while coasting would be a good thing.

Almost forgot.... In the Metro DC area, the most common following distance for a semi doing 55 MPH IS 8 - 10 feet. Of course if you're doing 55MPH on the Beltway, heaven help you, because the semis with be 5 feet from YOUR back bumper.

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"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?"

Except that this was also the same episode where they confirmed that chunks from a popping tire could kill you, and that under inflated tires are the ones that are likely to blow. Tires at max sidewall pressure are a good thing, better gas mileage, better handling, tires last longer.

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All cars are required to start in P & N. More than likely it is GM's sensor is misaligned. It happened to me once, if you lift up ever so slightly (toward reverse) the sensor lines up and the starts spins.

You waste gas by putting a stick shift car in Neutral and idling the engine.
All cars have overrun fuel cutoff.

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Not necessarily.

If the heat losses from the engine spinnning are less than the fuel burned to idle then this might be the case, but otherwise no.

Generally so if you can keep a steady pace going down a hill in gear go for it, but engine breaking will eat up a lot more energy on shallow hills or at high speed.

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Tried this last year returning from Florida, averaged over 33mpg with a full sized auto for 1,000 mile road trip. Average speed was approx 70mph. Noted an increase in mileage while drafting per on board display. Found that slowing down also works well, with a several mpg increase in slowing to 60 and 65mph. Be safe!

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