actual mpg measurements for "drafting" (not insight, though)

[dberns]

The television show "Mythbusters" takes various stories and tests them out to see if they could be true. For example, they hooked up a horse to the window bars of a (modern day rebuild of) a "western jail" to see if that Hollywood cliche could be true. (It wasn't).

They generally seem to have lots of fun confirming or busting the tales and stories.

In this week's edition, one "myth" they tested was whether "drafting" behind a truck did, or did not, improve mileage.

Short version: Yes, and quite a bit, but you'd have to be crazy or insane or suicidal.

Alas, they didn't use an Insight, just an, umm, I don't recognize the specific car and they didn't ID it. It's a full size sedan, possibly a small SUV. It's got the rear slope lines of a station wagon.

Anyway, first they did some crude tests using models in a wind tunnel. And eyup, they saw lots of improvment. So next step was a Real Big Truck (a new Freightliner with trailer) and the car traveling at 55 mph.

Net results:
a: baseline (car by itself): 32.0 mpg
b: drafting at 100 feet: 35.5 mpg, 11 pct improvement
c: drafting at 50 feet: 38.5 mpg, 20 pct
d: drafting at 20 feet: 40.5 mpg, 27 pct
e: drafting at 10 feet: 44.5 mpg, 39 pct
f: drafting at 2 feet: 41.0 mpg, 28 pct

(They guessed that maintaining the two foot distance
was just too harrowing, so the driving style was messier).

Anyway, these aren't from an Insight, but they do pretty
much show that drafting does work. But you'd have
to be pretty insane to do any of the close numbers.

[james]

IIRC the rule of thumb for safe following distance is one car length per 10 mph. (Correct me if I'm wrong.) The Insight's about 13 ft long, so that's about 75 ft at 55 mph. Spliting the numbers for 50 & 100 ft gives about a 15% improvement. Not too shabby

Though when I look at the distance at which the typical semi follows the vehicle ahead in moderate to heavy traffic, it seems quite a bit less than 75 ft.

Humm... I wonder if one of those laser measuring tools would give an accurate reading on a vehicle ahead?

[boogetyboogety]

Allow me:

a: baseline (car by itself): 32.0 mpg
b: drafting at 100 feet: 35.5 mpg, 11 pct improvement
c: drafting at 50 feet: 38.5 mpg, 20 pct
d: drafting at 20 feet: 40.5 mpg, 27 pct
e: drafting at 10 feet: 44.5 mpg, 39 pct
f: drafting at 2 feet: 41.0 mpg, 28 pct

Why don't we just develop a suction-cupped harpoon we can fire at the vehicle ahead of us designed to yield us infinite mileage? I'll file the patent today...

If (if!) I ever do draft a vehicle, it's a big square barn-door of a truck, and I've found by hanging back a second or so, I can get pretty decent mileage for a while. But the "harrowing" driving style is not conducive to pleasant driving, so I always drift back and... just... drive... I already have the most fun and fuel-efficient four-wheeled gasoline vehicle extant, so I'm a pretty happy driver...

[lesdit]

I tend to draft semi's pretty close, I'm guessing 30 ft. mpg is probably about +30mpg with that drafting. I have to keep a close eye out for cops. Also the stress of trying to maintain the distance is bothersome. If I pick a faster semi that is going more with the regular flow of traffic, or about 70 mph , they unfortunately tend to vary speed more, which adds to the difficulty of keeping in the air pocket behind the thing.

[red1dr]

I don't draft in cars because:
(1) it's dangerous
(2) it's stressful as noted above
(3) I can do better mpg driving slower than the trucks, coasting as much as possible in my Insight and doing 50-60 Pulse & Glide in my Subaru. I tried drafting in the Subaru and, according to the Scangauge, got about 29-30mpg in a fairly close draft. I can get mid-30's doing P&G, so why bother drafting?
(4) it damages the front of the car (nicks all over the front and pitted windshield of my Subaru from the days when I speeded)

It happens I have experience drafting trucks at 50-60mph, maybe 65 tops from my days as a bicycle roadracer:
(a) I drafted about a foot behind the rear tires because further back the draft dropped off rapidly.
(b) It was not a free ride, I was working hard just to hang in there and needed a big gear. If I dropped back more than a couple of feet I couldn't get back in. About ten feet back is a big downdraft of air coming off the top of the trailer.
(c) With typical box trailers like the one used in Mythbusters, a lot of air flows underneath the trailer. A trailer that goes down to the ground (moving van, some UPS trucks) worked much better. And a flatbed, tanker, or car hauler was fairly useless. A loaded truck was preferable because it changed speed more slowly. The data would depend on what type of trailer in involved.

This may sound dangerous but drafting is a major part of bicycle racing so we were very experienced at it. We would spend 4-5 hours within an inch or two of other riders' rear tires (touching meant a sure crash), and would sometimes motorpace behind a scooter at 30-40mph for training. So a foot from a truck tire was a larger margin. Drafting a truck (or motor home or bus), we would stay towards one side (usually the left) so we could stick our noses around to see the road ahead. And so that if anything happened we could quickly dodge around the trailer. One or at most two of us per truck so we could swerve freely if needed. It was great training because it made you work very hard, but we did it only when the occasion presented itself (or when we were battling strong headwinds).

Drafting with a car is far more dangerous because:
- your view ahead is blocked
- a car is much wider so you would have to swerve further to get around the trailer
- it's much harder to judge your distance from the trailer and whether that distance is changing because you can't see the front end of the car (a bicycle's front wheel is easily visible)
- to get the "meat" of the draft you have to be really really close, as explained above

So I don't draft in a car, though I'm delighted to benefit from the temporary suction of passing trucks!

[Waynetc]

The car mythbusters used was a Dodge Magnum Station Wagon. My Hunch is that using a car with a long roof and almost a Kamm back like our Insights, did help boost the FE, more than a sedan would have.

Note that the other part of the show included two other auto experiments 1) could a car enter a moving semi with a lowered ramp at 55 mpg (referencing the old show knight rider) and

2) Could a tire blowout on a semi decapitate the head of a driver in a car near it.

I won't spoil whether the myths were busted or not...but I will never draft a truck again.

Cheers
Wayne

[james]

You all seem to be missing the points I was trying to make, which are A) you get a significant effect from "drafting" at what is considered safe following distance; and B) you get even more effect at typical real-life vehicle spacing (at least on northern California freeways when traffic is heavy but still flowing freely). Those being the case, and since you are going to be following SOME vehicle at that distance (because if you allow significantly more following room, someone will soon cut in front), then why not follow one that'll give you an mpg boost?

Even aside from any drafting benefit, if given a choice of vehicle to be behind, I'd pick a semi. The drivers are (mostly) professional, thus less-given to hasty actions; since they're heavier, they take longer to brake than smaller vehicles; they're generally going a bit slower than traffic in the fast lanes; and I don't have to worry as much about some cell-phone using Hummer driver not noticing the small car in front of him.

[Martini]

Those are some really cool stats. I always draft the big rigs, usually about 20-30 feet back. My wife freaks if I get much closer. As long as I'm not in the middle lane, I'll draft almost anything bigger than me, with one foot nervously poised over the brake pedal. I never draft over 100 kph. Just too scary. I'll probably look back on these drafting days in a few years time and think, "I was crazy back then!"

[red1dr]

Originally Posted by james

You all seem to be missing the points I was trying to make, which are A) you get a significant effect from "drafting" at what is considered safe following distance; and B) you get even more effect at typical real-life vehicle spacing (at least on northern California freeways when traffic is heavy but still flowing freely). Those being the case, and since you are going to be following SOME vehicle at that distance (because if you allow significantly more following room, someone will soon cut in front), then why not follow one that'll give you an mpg boost?

The point you're missing is that you can gain more by slowing down and coasting wherever possible (at infinite mpg) then you gain by drafting and having to keep up with a truck going 65-ish, where you cannot coast. Is there a significant effect from a truck draft? Of course there is a measurable effect, but you can do better than that with other methods.

In Insight numbers, my 68-mile commute yesterday yielded 102.2mpg (despite a headwind on the way in) and I wasn't drafting behind trucks. Most of it is on I-95. Drafting a truck at a reasonable distance would not yield numbers like that - maybe at 2 feet but I'm not sure about even that.

But hey you have to decide for yourself what you want to do.

[james]

"The point you're missing is that you can gain more by slowing down and coasting wherever possible (at infinite mpg)..."

Sure, you can do that, IF your route allows. Kind of hard to coast on the flat, though

However, it seems contradictory to oppose drafting on safety grounds, while advocating going slower than the prevailing truck/slow lane speeds on a busy road. That just trades one dangerous behavior - one where you can control the degree of danger - for another behavior which I would guess is even more dangerous. I would be willing to bet that there are a lot more accidents caused by slow-moving vehicles than by drafting.