Drafting: stream of cars vs semi
 

I've been driving about 132 miles per day on a long stretch of highway for about a week now and I've noticed that drafting behind a stretch of normal consumer vehicles (cars, trucks, suvs) may be helpful for increasing fuel economy. Consumer cars are also generally going faster than semis so drafting has a larger effect. I've found that even being 3 seconds behind any vehicle is better than hitting the air alone.

Any thoughts on what is better for fuel economy? I admit that while driving behind other consumer vehicles I tend to pulse and glide more behind a stream of cars due to the large variation of speeds, which may attribute to the increase in fuel economy. I realize this will depend a lot on the type of car driven (larger cars probably have more to gain behind a semi) but if all cars were equal in size and drafting in both scenarios were at the same speed, would it be more beneficial to go behind a semi or a stream of cars?

I've wondered about that before. Specifically, I wonder if a whole line of cars causes a corridor effect, where the cars sort of carry the air between them. That would seem different from a single vehicle drafting another single vehicle, which breaks the wind and then allows it to reassemble behind.

I've noticed the corridor effect on my commute. The freeway I travel has 4 to 5 lanes going my way, so there are more cars pushing the air than on a two lane backroad, for example. I noticed the effect when there are fallen leaves or debris on the road. I noticed the leaves are still being blown in the draft when I'm 6 or 7 seconds behind a group of cars. That's a big gap.

The tradeoff to drafting behind a group of cars is that they tend to change speeds too unpredictably, which makes me hit the brakes sometimes, which wastes any gas I save from their draft.

Also think about ride height of the semi's vs cars. Semi's have a lot of air going underneath them only to hit the nose of the average car. Unless you are driving a lifted jeep cherokee with 33" tires then maybe a Semi will offer a better draft. I think that you are right with saying that a steam of cars on the highway offer a better draft for your situation.

A truck has a much bigger, squarer back end. The wake it leaves is much larger.

You may have a point about the under-truck airflow, though.

Personally, I prefer to go a little slower than the flow, and catch a momentary boost every time someone passes me.

My impression is that a column of cars produces a smaller, less turbulent, and possibly faster draft stream than a lone truck. If there is little crosswind, I'd prefer the cars. With greater crosswinds, I think the bigger hole left by the truck is more effective (depending on drafting distance, etc.)

Now... if you can find a column of trucks, you'll be cooking! :thumbup:

Yeah and even a couple of trucks side by side is really great. They actually get the air moving in their direction behind them. I find it really interesting how as a big vehicle comes up beside me how my car first turns into their lane as their bow wave air pushes my cars back end sideways. Then as they pass my cars nose gets sucked into their wake. The force is strong with trucks . . . master.

I like to be in somewhat of a "v" formation with a truck. I like to be beside them, with my front bumper just about even with the front of the trailer. I am easily visible in their mirrors, and I'm not tailgating. My SG reports that staying in that location is just as good as following 2 - 3 stripes behind.

-Jay

I've noticed since I put a vacuum gauge in my '88 that I get some benefit from semi's at 100-200 ft. behind them. It actually seems better at approximately 100 ft than if I was 2 or 3 car lengths behind them. I think that you should benefit from a string of cars. I remember as a child riding a bicycle and when a car would pass it was almost like someone behind you giving you a push.

:eek: 2 or 3 car lengths is very close. Even 100 ft is very close. That's a 1 second following distance at 70mph.

Have you experimented with longer following distances? Others report 2 to 3 seconds distance as having the strongest draft pull.

Come to think of it, now that I've done the math, when I count off a 3 second follow at 70mph it sure doesn't look like 300 feet but the calculator (and google) doesn't appear to be wrong...

3 seconds - about the time it takes to fully speak, "American Automobile Association". ;)

My thoughts exactly. I like having two slow moving trucks side by side and me behind.

That happened to me last night, only it was even better. I had a chartered bus in front of me going about 75mph and a semi to the right lane in front of me and a semi behind me. I was able to hit about 39mpg average for about 5 miles of this before it all fell apart. The total trip, however, was about 50 miles at the time so I imagine that I was getting even better mpg instantaneously.

This morning I was going about 65mph behind an RV and I only got up to 37.1mpg after driving 15miles, the total trip being about 50 miles again.

I'm starting to think that vehicles that are lower (like minivans) are better for drafting than semis!

There are plenty of container trucks on the 710 long beach freeway here in socal. Most do 55 or slower. Its best when i am behind one, one to the left of me and one behind. I am in the cradle, so to speak. I can eoc in the draft and helps get me to max mileage. On the 405 are mostly passenger cars. I just go 57 and set the cruise control and wait for something to draft.

3 stripes is my minimum, which is very close to my stopping distance between vehicles, about 130 feet at 60 MPH.

I am learning more with the Insight, because it has the bar graph that shows me instantaneous fuel mileage.

Best for me is a large motor home with the rear lower portions that extend down close to the ground. After a motor home the best big rigs are the tandems that have a hitch mounting point for the second trailer that extends down much closer to the ground.

The other big rigs seem to have much more airflow under the trailer.

I drive the speed limit, which means almost everyone is passing me. I just wait for the best opportunity for drafting. With the .25 CD of the Insight drafting is not as important as it was in the VX, so my drafting is much less agressive than it used to be.

3 stripes in the right lane also gives me the escape route to the right on the shoulder of the road if I encounter a large piece of debris, without having to worry about traffic blocking my escape route.

regards
gary

When I watch NASCAR (who also is big into drafting), it seems like it is only necessary for two cars to be together to get some good draft action going.

-BC

At NASCAR speeds, NASCAR following distance, and probably helped by NASCAR aerodynamics (especially that rear spoiler), the effects of drafting are highly exaggerated as compared to the effect of highway drafting.

Also, although I mentioned following distance, some here have measured a less effective draft when tailgating as compared to a reasonable distance; I suspect that NASCAR following distance only works with race car aerodynamics and highway cars suffer at that distance.

Today while EOCing downhill into my exit ramp, a tailgating big rig passed as soon as I entered the exit lane. I definitely felt the aerodynamic effect; it felt like I hit a heavy gust of head-on wind and slowed the car, but I think my butt dyno may have been misadjusted. I felt it around the time the steer axle of the truck was somewhere next to my car.

Several have mentioned having a in front and a truck behind them, as well. Do you have a death wish?? Saving a few oz of gas is nice, but is it worth getting squashed like a bug? Truckers enjoy great visibility (assuming the driver is alert enough to use it!), but a semi simply cannot stop as quickly as your car. If a pileup of any sort occurs, you're toast.

In a more general sense, many hypermiling techniques will produce accelerated wear and tear on your car. Anytime you stop the engine, you're cycling the ignition switch, starter or clutch, and collapsing the oil film that keeps your engine alive. Even a warm start will generate far more wear than simply leaving it to idle.

Evey time I see a video of King Hypermiler Wayne, I think, "That guy has obsessive-compulsive issues!" (He jams the car in into drive immediately after a cold start; he putts down the road trailing a dozen pissed off commuters, etc.) Yet time and time again, I see people wanting to do it just like he does, to achieve the same level of FE. Think people, THINK! As with anything in life, blind pursuit of an objective seldom works out in the long run. Go save gas, but try not to destroy your car, yourself, or the sanity of your fellow motorists in the process! :p

Actually, that particular technique doesn't sound so bad. If a pileup occurs, the trucker in front has great visibility and can't stop as quickly as your car. The trucker behind has better visibility than if you weren't there and he was closer to the other truck. Therefore you need only stop as quickly as the front truck, and the rear truck will be able to stop without crushing you...assuming the driver isn't "livin' on reds, vitamin C and cocaine".

That said, you still won't catch me doing it. If the feces does hit the multi-bladed air circulating machine, the consequences are dire.

Is that the extremist who is often shown on the news with his stock Honda Accord getting something like 75mpg?

[Stern mother voice]
What's all that potty talk young man?
Do I need to wash your mouth out with soap?
[\Stern mother voice]

Yea, that's the guy. Wayne Gerdes.
He has a pretty good site, similar to Gas Savers: https://www.cleanmpg.com/

-Moo

I too have noticed a negative push right when the tractor passes my car. It's somewhere between the nose and the tractor-trailer gap. I like to swing a little wide right as he begins passing, and then return to my position once the trailer is beside me.

Wayne Gerdes - you have to filter those news reports though media sensationalism. I've driven with him, and it's nothing crazy at all. He is driven and focused, but not crazy.

I think it's a kind of "bow wave" of denser air. Seems to me to come back at an angle from the front of the tractor, so depends on how far over in the lane you are. If he's keeping to the middle of his lane, you can go on the right hand lines of yours and it's a lot weaker and further back, whereas if you're hanging left and he's hanging right it can toss you all over the place...

I noticed however, that after I put turbulation strips on the back end of Marvin, this effect was MUCH reduced. I didn't see any MPG gain on that due to the tranny screwing up and finally crapping out. But it felt like I was a lot less affected by crosswinds, and last tank was on the high side despite mechanical issues.

I remember watching Mythbusters. They did a thing on drafting for better MPG.

Big Rig Myths (June 6, 2007)
In Exploding Tire of Death, Adam and Jamie try to find out if an exploding truck tire can kill. Meanwhile, Kari, Tory and Grant investigate another fuel efficiency fable.

Drafting behind a big rig will improve your car?s fuel efficiency.

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.


Interesting episode. I don't remember what the benefits were at a safe distance. I'll watch it again tonight.

, Bill

Testing by members here and elsewhere has shown that being too close is worse than not drafting at all. Reports of best draft are between 1 and 3 seconds following distance.

Iv felt, and measured this effect (with SGII) a few times. I was driving with 10 to 15 cars all doing 70 mph I was getting mileage similar to myself at 60 or so. I thought something was reading wrong!

Here in Socal the 710(Long Beach) is the mecca of container trucks. Usually thousands are going each way and you almost cannot avoid them. To exit from the fast lane is a challenge because they tail gate each other at a very small distance. The best way is to get between them early so you really have no choice but to draft. They even occupy the next to the fast lane if it gets to slow. Just have to make the best of a crowded situation.