Does following (drafting) a semi really save gas?

[Black Bart]

Gary stop and think about it just one second.
How can I hit you with stone if you are not tailgating.
For some reason this seems to escape you. If you don't tailgate you don't put your shelf in a position for that to happen.
I will not respond to any more comments because it is not going to change your mind so think what you will and enjoy your day and happy tailgating to you.

 

[saaber1]

Originally Posted By: Gary Allan
...is the "me" that wants to force others to drive the way they think others should. It's just the main ingredient in road rage.


This comment is really true. It got me thinking to a trip I took a couple weeks back on a two lane freeway where a semi was going regular speed in the right lane and some self appointed speed policeman in a white pickup was holding everyone behind him hostage in the left lane. He was driving the same speed as the truck and wanted everyone behind him to do the same.

We were mad to say the least at this idiot. Fortunately the semi driver recognized that this was dangerous and someone was going to do something stupid soon so he speed up and let everyone pass the white truck on the right. Very professional and everyone waved to him as they went buy in thanks. Everyone waved to the white truck driver too but it looked as if everyone suddenly lost four of their fingers.

 

[Gary Allan]

Relax, pal. I don't "tail gate" and if you've really read my posts, I don't recommend it. I can manage about 60'+ and manage a good assist from a big rig. I only have to have my nose inside the envelope to save LARGE. I've gotten stone chipped in my minivan without being anywhere close to tailgating.

We're merely bringing to light the real reason that you don't like it ..which is ..that you don't like it. I suspect my 60'+ would annoy you too, and you would react the same way even though there have (probably) been 100,000 instances of 60' proximity vehicles, at high speed, that didn't bother you at all if you were forced to endure it due to traffic density. I think that you just didn't like being tailed when you didn't think that they had a reason to do so. It bothered you.


..and that's fine with me

 

[leftlane]

I have yet to see another trucker get bent out of shape when another trucker tailgates or drafts behind them within 20ft and I see them do it all the time down the highway at 65mph+ for miles. Just seems to be the smaller vehicles that hack them off. Im grateful for the considerate truckers but it seems the majority of truckers are either hostile tailgaters or they rudely block traffic for miles driving beside other vehicles below the speed limit instead of being considerate of other drivers and passing then getting out of the way back into the right lane.

As for me tailgating truckers I dont do it. I see new truck tires on the highway everyday so I dont want to be close enough to catch one with my car or windshield. I was passing beside a trucker one time when one of the tires blew up beside me. Small pieces of it hit my car but thankfully it didnt do any damage, however it sounded like a shotgun going off when it blew up which scared me enough to almost swerve the car off the highway. I pass them faster now ever since then because I just dont want to be near one when it goes off.

 

[Gary Allan]

Quote:
Im grateful for the considerate truckers but it seems the majority of truckers are either hostile tailgaters or they rudely block traffic for miles driving beside other vehicles below the speed limit instead of being considerate of other drivers and passing then getting out of the way back into the right lane.


I don't see this too much. Now you don't want to lose what momentum you have if you're approaching a long grade with a slower truck in front of you ..and there's sensible use of the left lane if you're in congested traffic and there's a series of entrance/exit ramps in the upcoming expanse of roadway. Around here, if you hit the bypass at a certain time, you're effectively in high speed grid lock for the next 20 miles ..with progressive slow downs for exit:merge traffic. Too many vehicles on too few lanes.

His reaction is probably more akin to "you weren't invited".

 

[Bror Jace]

I'm disappointed with Bart's last couple of posts. I had respectful but specific questions about distance, etc ... that he didn't answer.

I'd also like to know how often he's had an economy car that was following him hit the back of his rig. A handful of times in his career? Once a year? Most into 'drafting' are very aware drivers ... as opposed to the countless thoughtless drivers out there with a myriad of bad habits that are the cause of most accidents.

"Just seems to be the smaller vehicles that hack them off."

Yes, after this and other discussions on the same subject, I have to conclude that it is an emotional thing, all out of proportion with the safety 'threat' posed by conscious drivers of economy cars following 20 ton behemoths. I'll continue to believe this until someone can calmly demonstrate otherwise.

I've driven abut a half a million miles in about 2 and a half decades and have been close to only 2 truck tires 'exploding.' They sure get your attention ... but I'll still follow a truck (or van, bus, camper, etc ...) at 60' without worry. Often, in heavy traffic, this kind of spacing is normal anyway.

 

[saaber1]

Originally Posted By: Bror Jace
... follow a truck (or van, bus, camper, etc ...) at 60' ...


Found this wikipedia article that shows the mythbuster results (thanks to everyone who reported about that mythbusters show, first I had heard of it, BTW you tube shows tests done in Madras OR, it's in the middle of nowhere, bizarre)

http://en.wikipedia.org/wiki/MythBusters_(season_5)#Drafting_For_Money

So if we assumed at 60 ft. a car was 18% more efficient (guestimate from the data in the article), and that a prius which normally gets 48 mpg at 55mph (the speed of the test), mpg would increase to approx. 56.6 mpg.

Having driven rented priuses (Prii?) for thousands of miles, I have seen massive mpg swings depending on the wind (as much as 11 mpg), so I think that particular car may be more influenced by wind than others Also it has a CVT which I assume would benefit more from drafting than a regular automatic or a manual.

Maybe those factors would push the gains at 60 feet beyond 18%? Not arguing for or against drafting, just trying to determine the real world potential gains (and losses such as safety and debris damage).

 

[ekpolk]

Originally Posted By: Black Bart
Originally Posted By: rationull
With an MT the crankshaft is mechanically connected to the drive axles with no slippage. How can a different load possibly result in different engine speed at the same wheel speed?
IT CAN'T CHANGE NOT POSSIBLE


True enough, but the amount of fuel burned to maintain the torque necessary to maintain the fixed rpms can very well change. If the resistance of the air in front of the vehicle changes, the amount of fuel necessary to maintain a set level of rpms will drop.

 

[ekpolk]

Crazy drafting (the sort that's patently unsafe and just shouldn't be done) can actually result in a large reduction in fuel consumption. Of course, IMO, and I think the "majority" analysis would be that the savings are simply not worth the increased risk of collision or vehicle damage from debris.

On the other hand from two years of Prius driving (with a glaring data display always in your face), I've found that even "moderate drafting" (the sort done at a respectful and safe distance) can still make a difference. I have found that 18-wheelers, full-size pickups, vans, Jeeps, etc. punch such a ridiculoulsy large hole in the air, that even following them at a good two second inverval, there's still enough disturbance of the airflow, that my mpgs will go up about 3-4 just by slipping behind such a vehicle (again, at a safe following distance). Presumably the increased velocity of the burbling air decreases its pressure and slightly reduces the air resistance of the vehicle penetrating the burbling air.

It works like clockwork -- I slip in a sane distance behind (not anywhere near up close) some other vehicle that's an aerodynamic disaster, and my mpgs "magically" float up by anywhere from two to four mpgs. The effect rapidly diminishes as you get maybe 5-6 car lengths back from the aero-disaster vehicle.

 

[rationull]

Originally Posted By: ekpolk

True enough, but the amount of fuel burned to maintain the torque necessary to maintain the fixed rpms can very well change. If the resistance of the air in front of the vehicle changes, the amount of fuel necessary to maintain a set level of rpms will drop.


I think Black Bart's contention (and the one I was replying too as well) was that you could tell when you were in the "drafting zone" by watching your RPMs drop while going the same speed. The original poster of that idea specifically said he was talking about MTs IIRC.

 

[ekpolk]

True.

 

[Bror Jace]

So, ekpolk, care to put a number on your distances?

I was in rush hour traffic this morning on I-90 going through Albany, NY and the typical distances (fore and aft) between the vehicles averages 30'-40'. You couldn't really get away from this sort of formation.

I used the number 60' above ... but no one's said whether they thought this was 'too close' or 'tailgating.' And, the reason I think they haven't is because it's no big deal.

 

[glennc]

There's a difference between 30'-40' when you can see through and around the car in front of you and 30-40' behind a semi. I get nervous within maybe (guessing) 100' of following a semi, which is about a second at 65mph.

Also the comfort distance for me depends on the traffic level. If traffic is light and well spaced and you're 1-2 seconds behind a semi, you're looking conspicuously close.

 

[Audi Junkie]

http://en.wikipedia.org/wiki/Tractor_beam

 

[Gary Allan]

Quote:
I get nervous within maybe (guessing) 100' of following a semi, which is about a second at 65mph.


Yes, if the semi is standing still. This is where I really check the rational from the truck drivers who object. Just when were they EVER the masters of "stopping short"? If anything THEY are the ones who really have to watch out for cars being too close in front of them. Physics just can't handle wishful thinking or personal preferences in terms of "proof".


Quote:
Also the comfort distance for me depends on the traffic level. If traffic is light and well spaced and you're 1-2 seconds behind a semi, you're looking conspicuously close.


Absolutely! It's a matter of perception. Not anything of real value in judgment. If you're comfortable in dense traffic at a given proximity and uncomfortable at the same or even greater proximity in lower traffic density...it's PURELY and emotional state. Not rationalized for the "truth" it ..but merely how you "feel" about it.

 

[glennc]

Originally Posted By: Gary Allan
Yes, if the semi is standing still. This is where I really check the rational from the truck drivers who object. Just when were they EVER the masters of "stopping short"? If anything THEY are the ones who really have to watch out for cars being too close in front of them. Physics just can't handle wishful thinking or personal preferences in terms of "proof".

I agree completely - with a caveat. (Is that not really agreeing completely? Okay, maybe not. But, hey, I'm trying!)

Unless he catches me napping or fiddling with the radio it's not the semi itself stopping short that I'm concerned about. It's general road debris that the semi can straddle. (Or not - if he hits something big and I have to drive through the shrapnel I suppose it's just as bad, except maybe I'd have some warning). A dead animal, the remains of somebody's lawn furniture (seen that twice!), miscellaneous twisted chunks of metal, or tire carcasses come to mind. Once you get up fairly close you don't have time to react, and these types of things can cause a wreck either if you hit them directly or if you don't have time for the correct reaction and cause a crash while attempting evasion.

Originally Posted By: Gary Allan
Absolutely! It's a matter of perception. Not anything of real value in judgment. If you're comfortable in dense traffic at a given proximity and uncomfortable at the same or even greater proximity in lower traffic density...it's PURELY and emotional state.

Can't agree at all on that one I'm afraid. Risk in traffic is constantly variable and we adjust for it by altering our following distances, among other things. In close traffic everybody probably accepts that they are encountering additional risk, just as they do in poor weather conditions, for example. When not in close traffic, should you maintain the same close following distance that might be acceptable in dense traffic, you are by election accepting an unnecessary additional risk and subjecting those around you to an additional risk as well. Whether the risk is worth the benefit is open to discussion, but it is not merely a matter of perception. The risk exists.

My humble opinion.

 

[lovcom]

When I'm not following a big semi truck on the freeway, my 2007 Yaris gets around 48 MPG on level ground, at 65 mph.

When I draft a big semi on level ground at 65 mph, my Yaris gets 78 mpg. I position my car about 3-4 car lengths behind the truck

Yes, drafting does work, and very, very good!

 

[lovcom]

FFRacer, your comment about the RPM's changing as proof of gas savings is complete Bool Sheet!

Most automatic transmission have a lock up torque converter, and manuals of course have a non-slip clutch, so the RPMs stay constant and the same, regardless of the MPG loss or gain.

Now what does change is fuel consumption.

 

[ekpolk]

Originally Posted By: Bror Jace
So, ekpolk, care to put a number on your distances?

I was in rush hour traffic this morning on I-90 going through Albany, NY and the typical distances (fore and aft) between the vehicles averages 30'-40'. You couldn't really get away from this sort of formation.

I used the number 60' above ... but no one's said whether they thought this was 'too close' or 'tailgating.' And, the reason I think they haven't is because it's no big deal.


Actual distances are hard to say. All the more so, since I change my "rules" depending upon the conditions. I'd summarize my position on all this as: First off, under NO circumstances will I ever engage in crazy-close hard-core "drafting". My life, and the lives of those around me, are worth more than the few mpgs I could gain. Second, I judge distances by time (one-potato, two-potato, etc). Third, any time I get so close to the rear of a vehicle, particularly a large truck, that I notice that HIS rearview mirrors are no longer visible, I back off some. The meaning of that sign you see on the back of some 18-wheelers ("If you can't see my mirrors, I can't see you") is not lost on me. Finally, I apply a rule I learned in flight school 25 years ago -- "if there's doubt, there's no doubt". If I have any doubt about whether I'm too close to someone, I assume that I am too close, and back off. Hey, I drive a Prius anyway, so I really don't need to draft to generate brag-worthy mpg numbers!

 

[ekpolk]

Originally Posted By: lovcom
FFRacer, your comment about the RPM's changing as proof of gas savings is complete Bool Sheet!

Most automatic transmission have a lock up torque converter, and manuals of course have a non-slip clutch, so the RPMs stay constant and the same, regardless of the MPG loss or gain.

Now what does change is fuel consumption.


And if you have some form of scan gauge, you can see this in crystal clarity. Oddly enough, the unique CVT in my Prius does actually vary engine rpms on an almost constant basis, even when you're constant speed, since the ECU is constantly adjusting the gas/electric balance to optimum, based upon a zillion factors, including critically, the charge state of the battery. But in a conventional car, at a fixed speed, engine rpms will stay constant (assuming no downshift). The fuel flow and torque required to maintain that rpm WILL, however, shift depending upon many factors, including wind resistance present at any given moment.