Drafting = 3 mpg increase

to draft you must be within a few car lengths to get the "sucking" effect. but to get the benefit of reduced air resistance, you can be 4-5 car lengths which is more than can be maintained on a busy freeway

 

<div class='quotetop'>QUOTE(electricity_guzzler @ May 25 2006, 01:07 PM) [snapback]261025[/snapback]</div>

It works even better if you can snag the car ahead of you with a bungie cord. And never mind about the collision you'll cause one day--all that really matters is that you saved 25 cents on gas.

 

With the size of the turbulence behind a semi, I'd imagine you could follow at a sizeable and "safe" distance, i.e. 3 lengths, or more, and have as much drafting benefit as following a normal SUV at 2 lenghts, or so..

Obviously, we ought all be safe about it.

 

<div class='quotetop'>QUOTE(hobbit @ May 30 2006, 10:06 AM) [snapback]262691[/snapback]</div>

This is true. You can still get a 3-4 mpg increase at 4 to 5 car lengths back with the right atmospheric conditions (i.e. no strong crosswind). This is a much safer distance that greatly improves your odds to react to the unexpected.

 

<div class='quotetop'>QUOTE(hobbit @ May 30 2006, 10:06 AM) [snapback]262691[/snapback]</div>

Any suggestions on proper distance, 3 carlengths maybe, and for those who are uncomfortable with drafting, even 5 carlengths might offer some benefit? I know at that distance someone is likely to cut in, but whatever distance is close enough to prevent that but still be safe. Where there is a lot of traffic congestion anything greater than 1 carlength results in someone cutting in so in that situation just get behind an SUV and hopefully you can see through their windows to see brakelights several cars ahead. The nice thing about a semi is even when it hits other vehicles it probably doesn't slow down that much so you might be able to stop before hitting the truck if you have 2-3 carlengths.

Dennis

 

In Kansas, the legal following distance on the highway is 2 seconds under normal conditions. I'm sure the rules vary from state to state, but more or less fall in line with common sense. So if by proper you mean legal, there's one answer.

Yes, by increasing your following distance, you may allow some idiot to cut you off, but that doesn't mean that you yourself should be come another idiot on the road by tailgating.

Kansas Driving Handbook (PDF, 3.66 MB)

The Maine handbook (PDF, 1.01 MB) also uses the two-second rule.

 

It's about a car length per 10 mph, so it depends on speed. Then
again, think about what you'd do if the truck suddenly lost a
retread and it was skittering toward your nose in a somewhat
unpredictable fashion. How close would you want to be then? Happens
pretty frequently; you see those "alligators" lying alongside the
highways all the time.
.
_H*

 

The two second rule would certainly be safe. At 60 mph it works out to 176 feet or almost 60 yards. This might be the legal following distance but on congested highways it is not enforced, I am not one to tailgate, but I had never stopped to calculate the legal following distance. More than half a football field is no doubt safe, in most circumstances in traffic someone would fill that space. I think hobbit's rule of thumb of around 60 feet at 60 mph would be a little close for my comfort level. Obviously everyone's perception of "proper" is different. Five carlengths for the Prius is about 73 feet, this would work for me at 60-65 mph, but any closer would not, 88 feet would be a 1 second gap at 60 mph and is 6 carlengths. So proper following distance for me is 12-15 feet per 10 mph, maybe too much to get much drafting effect.

Dennis

I realize that I misread Hobbit's reply, I think I must have assumed when reading it that a carlength was 10 feet which is how I got the 60 feet. Sorry, I need to read more carefully before commenting. His rule seems dead on to me (though it is illegal, so is driving over the spped limit and I do that from time to time, though not so much on the highway anymore)

 

Yeah, yeah. It does increase your MPG, but the increased risk of windshield damage and paint damage makes drafting unwise. I've gotten 70+ mpg following car carriers and moving trucks, but I figure a new windshield would cost way more than the pennies drafting could save me at the pump. Even without drafting, I already average 55mpg doing 60-65mph on the freeway.

 

very interesting article about insurance fraud... drivers driving slowly on purpose trying to goad drivers into tailgaiting them so they could slam on the brakes and file a claim against ya...

 

I have a real problem with "drafting". It is not a safe practice. Most people think they are good drivers because they have been so many years without an accident, or because they drive slower than the "idiot speeders", or whatever reasons. From what I have seen however what we often have are lucky drivers as opposed to good drivers.

Tailgating trucks? Unless the trucker is aware that you are tailgating, and you have his permission, and you are willing to put your life in his hands (since all you see is the back of a truck), is an insane practice. It is right up there with riding a motorcycle without a helmet in a busy suburb.

There are more than potentially blown tires to worry about...... How about animals in the road, potholes, rocks thrown up by tires or debris blown off the top og the trailer? The concept of going 65 mph and only being able to see the rear of a big truck rather than what is ahead in the road just makes me squirm.....

If you and to draft them hop on a train and follow the car ahead of you J/K....

Seriously folks, when you draft on a road with people who are essentially unskilled you are risking a lot. Think of all the times that someone has interpreted "yield" to mean "speed up and cut someone off". Remember that in most states it is illegal to read while driving a car, unless you are reading a map. Which actually means that if you are completely lost is is ok to read a map while driving...

Additionally, the handling properties of the Prius, (not intended to be insulting), are basically non existant. It corners worse than nearly any car produced which means that if someting does happen while drafting you are far less likely to be able to avoid a problem than the average sedan.

 

There was a study out that I remember reading that stated you get the same benefit whether you are 1-3 car lengths behind a larger vehicle. After that it starts to drop off rapidly. Also, when you are next to a truck you will share the wind resistance like if you were one vehicle. I cannot remember if you need to be 1-2 car widths away to avoid this when passing. I also remember basically the same results in my aerodynamics classes 10 years ago.

I would not do it though. The extra couple of dollars in savings is not worth the possibility of an accident. There is work being done for auto sensors/self driving sensors to allow cars to convoy within inches of each other to draft and reduce traffic. I think this is still 15 years off. We have the technology to do it just not the infrastructure.

 

what's the drag coeffecient of a Prius? You know I have to get some technical numbers in here.

 

<div class='quotetop'>QUOTE(GasGuzzler87 @ Jun 20 2006, 03:26 AM) [snapback]273967[/snapback]</div>

0.26 I believe.

 

<div class='quotetop'>QUOTE(jdjeep98 @ Jun 20 2006, 05:45 AM) [snapback]274001[/snapback]</div>

wow, that's very good.

 

<div class='quotetop'>QUOTE(GasGuzzler87 @ Jun 20 2006, 06:33 PM) [snapback]274330[/snapback]</div>

Yup second best after the Insight.
Anyway, I'll admit to drafting, but I am far enough back to see the trucks mirrors. I've also noticed a benefit if I am in the next lane over from the truck, again avoiding The "No Zone". I also make sure that if I am driving along side the truck, I do not impede traffic.
I have also notice a benefit of "drafting" off of Hummers and the like....

 

Wow, this whole thread makes my skin crawl. I really hope this is a troll. As a former cop, firefighter, paramedic, truck driver, and (more applicably) emergency driving instructor, please allow me to ponificate.

The human body's reaction time takes about 0.4 seconds between visual stimulus and initiation of response. Highly trained people, such as drag racers (whose livelihood relies on the fastest response time) can get their times down to around 0.1 second - a tenth of a second. Then there's the time from initiation of response to actuation of the control mechanism (brakes or steering wheel). Once again using a drag racer as a definition of "the best", where they're sitting, adrenaline pumping, with the accelerator cranked and their foot waiting to pop the clutch, they still take a tenth of a second. Amateurs, taking their car to the drag strip, pray to get their reaction times down around 0.5 seconds - a half second - again, this is with adrenaline at full tilt and waiting on a known stimulus - the green light.

For the sake of argument, let's assume you're better than the amateur drag racers, and you're paying complete attention, and there's no delay at getting your foot from the accelerator to the brake. We'll assume your reaction time equals the initiation of a response, and it takes you 0.4 seconds to apply maximum brakes.

At 70mph, you're travelling 103 feet per second. At 80mph, you're going a tad over 117 feet per second. Therefore, at 70mph, you'll go 41 feet before you even start to brake, or 47 feet at 80mph. My 2005 Suburban is 18.25 feet long (the Prius is a bit over 14 feet). At 70mph, you'll go two to three car lengths before you even start to brake or swerve, then you still have to get yourself out of trouble. If you're one car length back, you have less than two-tenths of a second to respond at 70mph. It ain't gonna happen, no matter how good you think you are.

There's a danged good reason why the two-second rule exists. You have time to get yourself out of trouble if you stay two seconds back. If you choose to tailgate, I just hope you haven't killed anyone else when they cut your car apart to get you out.

Yes, this is harsh. No, I won't apologize. Think of the other people on the road - it's not all about you or whether you can save a penny or two.

 

Glenhead, please excuse us mpg-heads. I think everybody actually realizes that it can be unwise, and probably we talk about drafting more than we actually do it. Hope so at least!

It is the lure if the instantaneous mpg-gauge that makes us dumb in this way. To counteract that, I will suggest that it is hard to save more than 35 cents per hour by drafting. Under many circumstances, the risk will clearly outweigh that.

I propose, at least, a voluntary ban on drafting at night. Who'll sign?

My Prius windshield has two minor cracks (not as a result of drafting, I hasten to add) so that the next energetic windshield contact might just finish it off. So probably I'd better stay back...

 

glenhead. great explanation...i guess. i fully agree with your conclusion but your logic to get there is completely invalid.

what ever you are following has momentum as well. a truck much more so than a car. so after the reaction time is overcome, you will lose your momentum much quicker. we are not judging reaction times to a brick wall. its a moving object so the "following" distance will be much smaller than the "impact" distance.

now, after saying that, i still agree that drafting is completely stupid and in WA state, is against the law. there has been emphasis patrol for drafting going on for a while now and for good reason.