Following a tractor-trailor increase highway MPG?

I used to draft trucks but only "back in the day" when I had a CB radio and had permission to draft. I can't think of many activities that are more dangerous on public roads. My brother trucks for a living and he is fed up with people "drafting as they are often out of his field of vision and they cannot see around his truck to determine the safety of the road ahead.

And to think... Some people want to "ban" cell phones because they are too dangerous....

 

<div class='quotetop'>QUOTE(tnthub @ Jun 2 2007, 06:35 PM) [snapback]454062[/snapback]</div>

and as I would say the last thing you want to do is piss off a trucker because they have lots of friend's on the road and they send out on the CB "guy in the red prius is an a-hole" and will team together and block lanes and ultimately ruin your trip. Nobody likes a tailgater period.

 

<div class='quotetop'>QUOTE(morpheusx @ Jun 3 2007, 12:56 AM) [snapback]454234[/snapback]</div>

At least then you'll have an excuse to drive under 40mph So long warpstealth, hello stealth (and better mileage).

 

This week's Mythbusters tested drafting, and did find a significant improvement in mileage. They also showed what happens if truck throws a tread at your car from close range (breaks the window and decapitates the driver).

 

<div class='quotetop'>QUOTE(alanh @ Jun 9 2007, 11:21 PM) [snapback]459005[/snapback]</div>

I love the show...I saw this episode and sparked a debate at work... I also do not generally draft trucks, but here's a nice summary:

http://en.wikipedia.org/wiki/MythBusters_(...fting_For_Money

Drafting For Money

Myth: Drafting behind a big rig will improve your car's fuel efficiency.

Status: Confirmed.

To test this myth, the build team procured a car, a big rig, and a device that could measure a car's fuel efficiency. They then drove the car behind a moving big rig at various distances ranging from 100 to 2 feet and measured the amount of fuel the car consumed. The Build Team discovered that the closer the car was to the big rig, the less drag is produced, thus the more fuel saved. At just ten feet, the car managed to increase its fuel efficiency by 40%. Drafting at two feet was slightly lower than the ten foot distance, mainly because Grant had to keep working the car pedal to maintain distance from the truck. However, that did not dispute the fact that drafting actually can increase your car's fuel efficiency. However, the Build Team has warned that drafting is incredibly dangerous because the truck driver may not able to see you and you may not be able to react in time if the truck were to make a sudden stop.

Scale wind tunnel test: drag reduction from baseline at 50 mph: "seven car lengths": 21%; 10 ft: 60%; 6 ft: 80%; 2 ft: 93%

Controlled road test: fuel consumption decrease from baseline at 55 mph: 100 ft: 11%; 50 ft: 20%; 20 ft: 27%; 10 ft: 39%; 2 ft: 28%

Notably, they don't mention the Truck's mileage... as a side note, can you see truck mirrors from 100 feet?

 

As a semi driver, I haven't seen too many Priuses drafting behind me so usually they always pass me on the roads. However trucks have many blindspots and it bothers me when one is so close behind me where I can't see them in the mirrors. That's dangerous. There have been few times where some ppl would tailgate me so closely that I would slow down on purpose so that they can go around me. Last thing I need is an accident.

Turcks in most cases get about 5-6MPG at the most, whether they are being drafted or not.

And, there are many gravel trucks that tend to spit lots of rocks that will do plenty to put a crack in your windshield....I would never follow or even get near those trucks.

 

<div class='quotetop'>QUOTE(Dr Ed @ Jun 2 2007, 04:44 PM) [snapback]454050[/snapback]</div>

Over 87% of all items are delivered by truck. Would you really like to see the costs increase on every thing you buy?

You must be following some owner operators because company trucks are usually well maintained and the ones with a name on the side cannot afford to invite damage claims or lawsuits.

Insist to your congressperson that the Highway patrol do their job of safety inspections. It can make a nice source of revenue for the state.

(Traffic manager when not driving my Prius )

 

<div class='quotetop'>QUOTE(alanh @ Jun 10 2007, 12:21 AM) [snapback]459005[/snapback]</div>

I'm going to start wearing a helmit with mylar vest, then
And ... In the same vein, I gotta get a sun roof installed ... that way I can sling my grappling hook onto their bumper more easily

 

I tried this on a long trip. It does help a bit but I think the drawback is the erratic speeds of the trucks. It is difficult for them to maintain a constant speed going up and down hills. I prefer the cruise controll and maintaing a 60 mph constant vs the dangers and up and down speeds of trucks. Drafting is not worth it in my opinion.

 

<div class='quotetop'>QUOTE(hill @ Jun 16 2007, 01:52 AM) [snapback]462880[/snapback]</div>

Bob Wilson

 

<div class='quotetop'>QUOTE(garyg @ May 31 2007, 11:42 PM) [snapback]453026[/snapback]</div>

I had a car that had this feature, minus the graphs the Prius uses. It was a Le Baron convertible...

 

<div class='quotetop'>QUOTE(brad34695 @ May 29 2007, 06:04 PM) [snapback]451714[/snapback]</div>

Yes, it does.

First off, I have a master's degree in aeronautical engineering, so I have some technical expertese in this area.

The reason is that the truck leaves a disturbed airstream, in which some of the air is moving in the same direction you are, which effectively reduces your airspeed, lowering the energy needed to cut through said air. Let's call this the wake effect of the truck (as that's what an aeronautical engineer would call it.)

This effect is measurable (by precision instruments) for several hundred lengths of the truck, but is significant for drivers out to somewhere between 3 and 10 truck lengths. The closer one is to the truck, the more significant the MPG savings will be.

Additionally, cutting through disrupted turbulent air will decrease the energy needed to do so, relative to cutting through still air. This effect is smaller than the wake effect above, but not insignificant. At longer distances from the truck, it will equal and then surpass the wake effect, but when that happens, your mpg savings will probably be less than 1 mpg, perhaps a lot less.

Perform the following thought experiment. Take a Prius, with it's wonderfully low drag coefficent, and cut it in half through the cab, dividing the front seats from the rear seats. Now imagine these two half Priori driving down the road, far apart from each other. (Please don't worry about driving on a single axle, etc. - this is a thought experiment.) Each half will have a higher drag coefficent than the whole car, and require a lot more energy to push them at a set speed. Now bring the two half cars closer and closer, until they touch. The drag coefficent for the combined cars must decrease to the value of the uncut car, in a fairly continuous manner. What this tells us is that there are benefits for both halves from drafting, but probably a lot more for the rearmost body, even when they are of comparable size.

Now drafting off a truck won't make a huge bit of difference to the truck, but it might help a little, and almost certainly won't hurt the truck's mileage. What would help the truck more would be if it could draft off a cheese wedge shaped sports car which would break the air in front of it.

Now, having said all that:

1) Never drive in an unsafe manner. Be comfortable with your visibility and stopping distances. No amount of gas is worth risking injury or death for - it could be you, or some stranger who gets hurt or killed. As a driver on public roads, you have an obligation to drive in a manner that is safe to yourself and others. Don't shirk this obligation.

2) Driving right behind a truck is quite safe in terms of the truck stopping vs. you stopping. If the truck slammed on the breaks, and you did the same as soon as you saw the truck's brake lights, the much slower stopping of the truck would make it very safe for you, kind of like a running back with several huge offensive linemen leading the way for them. Anything ahead of you will likely get plowed away by the truck.

3) If the truck is likely to shed debris, that's a lot more dangerous than the truck stopping suddenly. You will have almost no time to react, and very little guidance as to what the right reaction would be. This is the real danger of drafting behind a truck. Of course, on a motorcycle, one has more room to manuver within one's lane, but the consequences of getting hit by a large piece of debris are much worse than in a car.

Hope this helps.

Regards,
aeronaut

 

The MythBusters recently did a bit on this called Big Rig Myths. Short answer, yes it does. A surprising amount if you do it right...

 

<div class='quotetop'>QUOTE(aeronaut @ 2007 July 28 5:14 PM) [snapback]486847[/snapback]</div>

I disagree. There's no margin of safety when you're drafting. What if the trucker happens to slam on the brakes for an obstacle you can't see, or the obstacle turns out to be a moose that suddenly appears in front of you from under his rear tires....? Either I'm gonna hang two seconds back, or I'm gonna pass him.

 

Hi..... looking over my old posts and came across yours...

Not too much opportunity to 'draft' unless I go on long trips. BUT, have you noticed all the chunks of tire along major interstates? Must be many TONS scattered along the roadsides. I agree with another poster in that the savings in gas is not worth the chance of getting ones' Prius smacked by a piece of rubber weighing several pounds, not to mention other debris kicked up by trucks. Prius body parts are darned expensive to replace. I also understand that road debris has been known to cause expensive to repair damage to the Prius trans axle gear case. (There is an aftermarket 'shield' that can be installed under the engine area to prevent this.)

Gary....

 

Like most of the people above have said... It does increase your MPG because the truck is directing most of the air over you. The decrease in resistance means that your car will be able to use less energy to maintain its speed. Use the "2-second rule" when following a truck though. If the back of it passes a certain tree or mailbox or whatever, it should take you at least 2 seconds to pass that tree, mailbox, whatever. Safety first, then fuel economy.

 

For drafting to make a difference, you have to follow to close to be safe. If you have a CB and the Trucker gives you permission, follow at you own risk. The Prius gets such great MPG, risking life and to limb to bump it another 1-3 is just plain stupid. DUH-DU-DUUUUHHHH!

 

Mythbusters tested this extensively. It was 6 car lengths max from a big truck trailer to be effective. Run cruise while slowely coming up behind a big truck and watch the HSI go up in mpg's. Works every time no matter what vehicle you are driving.

Mike

Mobile on my SGH-i717