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Discussion Starter #1
I know this topic can be somewhat controversial so let me say I am not suggesting un-safe driving practices but I want to share my experience in my gen 3 Touring while drafting other vehicles at highway speeds. Drafting means to follow other vehicles to take advantage of the reduced wind resistance behind the leading vehicle. Unsafe driving practices can happen when the trailing vehicle follows too closely (tailgating).



I initially tried drafting with my gen 2 Insight. While testing, I found that I could increase mileage by 2-3 MPG while traveling at what I thought was the minimum safe distance from the leading vehicle. I gave it up because I thought the level of attention needed to keep this minimum safe distance was too stressful to maintain, especially on longer trips.


Then along comes gen 3 with adaptive cruise control and some might consider it a bit of a game changer for safer drafting. I recently completed a 675 mile trip and had good opportunity to assess. For my test I configured ACC to the maintain the Honda configured minimum follow safe distance from the leading vehicle. When leading vehicle was lost (leaving highway), I attempted to acquire another vehicle to follow traveling at my target speed as soon as possible, but I maintained target speed of 67 to 70 mph throughout testing, including control testing. Testing was done for 1 hour under both scenarios under similar conditions.



MPG with drafting = 47
Control MPG (no drafting) = 43


I was happy with both results. Car was loaded with excess of 500 lbs of people, dog, and luggage. Also, my wife had her the electric seat warmer on the whole time!


I sought out larger vehicles such as Vans, and Trucks to follow. It was apparent almost immediately the car went into EV mode more of the time when drafting (and now no stress in keeping my distance from the leading vehicle). I realize drafting is not for everyone and there may be some risks that some people don't want to make and that's OK, but I thought my experience interesting enough to share with the group.
 

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I did not know about this whole drifting thing, I guess I am not very knowledgeable with physics. Thanks for sharing, the results are pretty impressive actually.
 

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Thanks for sharing KD. I was actually wondering about this topic myself. For those interested, here is a link to Honda's explanation of how ACC works. I've also attached a picture from the Insight manual (pg 492 is where the ACC section starts) on following intervals for each of the bars you choose with ACC. Given your comments, I suspect you were a little over 100 feet behind the car/truck in front of you while in the "one bar" mode of ACC.

https://www.youtube.com/watch?v=yUqNcnUf6gg#action=share
 

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It would be interesting to see how the MPG varies with following interval. I would guess that you see significantly reduced MPG benefits at 2 bars.

When I took driver's ed, they advised a 4-6 second following interval. So it is really interesting to see, in the Insight manual, that the "extra long" following distance is just 3 seconds. In Jersey if I ever tried to maintain a 6 second interval I'd just get merged in front of right away.
 

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Thanks for sharing @KlumzyDriver. Did you find you had enough visibility and stopping distance with the "short" interval selection? Also, was wondering if your test results are based on "Normal" or other particular mode (Econ or Sport)?
 

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Drafting definitely makes a difference.
The down side can be if the road s very dirty you can get grit on your windshield and paint.
On the highways in Canada I will often draft of doing long highway drives through rural areas, it’s actually safer in a way.
Driving at night I will draft behind a moose catcher (18 wheeler) if a moose runs out on the highway and you are in a small car you are toast, but the 18 wheeler will take it out!
 

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Discussion Starter #7
More than once, a car would manage to get between my car and the vehicle I was drafting or the leading vehicle would brake unexpectedly. My car would safely slow or even brake (also speed up) without my input to maintain the desired distance separation.


I noticed if I was fortunate I would be following a driver that (like me) anticipates traffic or other need to adjust speed so not to to engage either braking or heavy throttle more than necessary. I also drafted behind (for a very short time) one idiot who who seemed to ride his brakes every minute or two without cause to do so. I made kind of a game of it- selecting and re-selecting (better) target vehicles to draft behind.



I admit I was fortunate in my testing to have conditions favorable...light traffic but enough traffic to find target vehicles to draft behind traveling in my desired speed range. One does not always have optimal traffic conditions and I would not try it in rain-fog-snow, etc. Also, as was pointed out in another post, be aware of road conditions - where grit or sand exists on the roadway, I would increase vehicle separation so not to be the recipient of this material- common sense rules!
 

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I use ACC a lot, and I found it quite helpful in mpg, and also many other aspects. It makes driving much easier, as I'm not worried much about gassing and braking. I have seen that when I follow a car I get better mpg, but I was not thinking about this drafting phenomena, very interesting. My taught was when following the front car, we make more braking and battery recharge, comparing when we driver at same speed but with no car in front of us.
Despite all the good things, there are a few things that bothers me with this. There are always some maniac drivers (unfortunately many of them here in NJ) that try to push themselves between our car and the front car, especially at highway ramps in and out. Also the extremely sudden brakes from the front car could be dangerous.
good luck
 

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Despite all the good things, there are a few things that bothers me with this. There are always some maniac drivers (unfortunately many of them here in NJ) that try to push themselves between our car and the front car, especially at highway ramps in and out.
Yeah, do not rely on ACC if you are in the rightmost lane and people are merging. The car doesn't react to lanes to the left and right of the one you're in. It's a minor miracle that it can see the car in the same lane as you on a turn. They must be using the optical camera near the rearview mirror.
 

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Yeah, do not rely on ACC if you are in the rightmost lane and people are merging. The car doesn't react to lanes to the left and right of the one you're in. It's a minor miracle that it can see the car in the same lane as you on a turn. They must be using the optical camera near the rearview mirror.
Yep, ACC relies on the front sensor camera. In addition to the above watchouts for rightmost lane and merging, the attached pages of Owners Manual also cautions of detection limitations at curved roads, bridges, and when the vehicle in front of you has an usual shape (e.g. upper section of empty carrier truck). Good reminders to keep in mind while chasing fuel efficiency.
 

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Discussion Starter #12
It would be interesting to see how the MPG varies with following interval. I would guess that you see significantly reduced MPG benefits at 2 bars.

When I took driver's ed, they advised a 4-6 second following interval. So it is really interesting to see, in the Insight manual, that the "extra long" following distance is just 3 seconds. In Jersey if I ever tried to maintain a 6 second interval I'd just get merged in front of right away.

Mgldan,

The process of what is called "hypermilling" is to seek multiple ways to achieve MPG far beyond the car's official EPA mileage ratings. One way some overly aggressive hypermiling drivers do this is to draft closely behind a leading vehicle, getting "pulled" along by the low air pressure created immediately behind the lead vehicle. For obvious reasons, this is a VERY dangerous practice to be avoided but when you hear a story about a driver getting an absurdly high MPG, this is one way they may attempt to achieve it. I don't mean to give a "black eye" to all hypermiling drivers; most drivers follow safe driving practices and do not tailgate. Reasonable hypermiling practices do help in obtaining better mileage.



This turbulent low air pressure "bubble" has it's greatest effect when tailgating closely (please, please, please don't try this!) but it quickly dissipates with increasing distance behind the lead vehicle. My test was only to simply see if drafting can be done SAFELY and also determine what impact it would have on mileage using ACC, To address your comment, I did try drafting at 2 bars following distance but it was too difficult with other drivers repeatedly merging themselves between my car and the vehicle I was attempting to follow so I abandoned the 2 bar test. Also I did notice in the short anecdotal 2 bar test that EV node did not come on as frequently as using ACC and one bar. Drivers should never follow other vehicles too closely and and remember ACC is no substitute for common sense, attention to the road ahead, and driving the way they most feel safe (which may not include drafting).
 

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There’s no doubt that drafting increases mpg. There’s also no doubt that it’s not safe. Besides the obvious shorter breaking distance there’s always the unexpected. Road debris is one that comes to mind for me. And where do you think those big pieces of rubber on the road come from? Maybe the truck you’re drafting?
 

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Thank you for a very interesting thread, a good read, ...though lots of safety worries.


I think of "drafting" as mere inches separating the front bumper from the rear bumper of the car ahead. If ya drive that way you'll increase mpg alright, as demonstrated on the NASCAR circuit; but as stated above, its very very dangerous, and carries severe legal penalties for doing it, aka "tailgating."


I never tailgate, but I do use, lets call it a "normal or reasonable following distance". On the Interstate on flat terrain I set the cruise control at 70, and make some mpg comparisons by resetting the trip computer under different wind/speed conditions. What I've found when I looked at this a long time ago was:


1. In low wind or no wind conditions almost no mpg improvement from following or not following a car. A small mpg improvement by following a van / simi or other tall vehicle.


2. In moderate head-wind conditions following a car or a tall vehicle can improve the mpg to the point it was at similar speed but in the no head wind condition. In other words, normal following distance can give enough benefit to negate the mpg decrease from a moderate head wind. I've found this benefit all the way down to around 40 mph.


I really don't think about this too much because I like to drive at a steady speed, with or without use of cruise control. I've found it hard to find someone to fallow, who both drives steadily and at a speed that is comfortable for me. So for the most part, I don't think about following someone unless there is a head wind.
 

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Mgldan,

The process of what is called "hypermilling" is to seek multiple ways to achieve MPG far beyond the car's official EPA mileage ratings. One way some overly aggressive hypermiling drivers do this is to draft closely behind a leading vehicle, getting "pulled" along by the low air pressure created immediately behind the lead vehicle. For obvious reasons, this is a VERY dangerous practice to be avoided but when you hear a story about a driver getting an absurdly high MPG, this is one way they may attempt to achieve it. I don't mean to give a "black eye" to all hypermiling drivers; most drivers follow safe driving practices and do not tailgate. Reasonable hypermiling practices do help in obtaining better mileage.
...Why was this addressed to me? I didn't say that I hypermile, and I also didn't say I condone or reject the practice.

You started the thread, and you said you used the minimum distance that ACC allowed, which is one bar. I asked how if you tried two bars. You said you tried that and didn't get numbers you're happy reporting. All of that is helpful to know. Thank you.

My second paragraph was meant as a side note. I was making an observation. The extra long following distance (four bars) is, according to Honda, not even a full three seconds. This when the practice I was taught is a minimum of four seconds.
 

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I don't fully trust the Honda sensing technology in the car so I like keeping a distance from cars in front and behind me. I have experienced for the first time today where the brake alert popped up even though no vehicle or object was ahead of me while moving at 40mph. I think it might had been a shadow on the road as I started to curve that probably confused the car.

Honda Sensing Problems - Honda Problems
^Honda sensing problems with other Honda vehicles.
 

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I don't fully trust the Honda sensing technology in the car so I like keeping a distance from cars in front and behind me. I have experienced for the first time today where the brake alert popped up even though no vehicle or object was ahead of me while moving at 40mph. I think it might had been a shadow on the road as I started to curve that probably confused the car.

Honda Sensing Problems - Honda Problems
^Honda sensing problems with other Honda vehicles.
Just be vigilant. I had what I thought was a false warning. It was a speed table on a road with the speed limit set to 35 mph. Car didn’t get hurt but it made me pucker up.
 

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Just be vigilant. I had what I thought was a false warning. It was a speed table on a road with the speed limit set to 35 mph. Car didn’t get hurt but it made me pucker up.
I guess your Insight wasn't ready to fly... :grin:

 

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So... speed bumps in NJ aren't painted? Kinda hard to see there's a speed bump when it's same color as road.
On certain streets in Boston, I have seen speed bumps not being painted, but there is a street sign warning about the bump. It's rare though so I assume the same in NJ.
 
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