“Be kind, for everyone you meet is fighting a hard battle” - Often attributed to Plato but likely from Ian McLaren (pseudonym of Reverend John Watson)

Saturday, June 16, 2007


In thinking about what I've done to attempt to minimize fuel consumption there are three things that stand out in my mind as potentially dangerous. I say "potentially dangerous" because for all three of them it is "common knowledge," stated all over the web, that they are. For the sites and blogs that I frequent (devoted to fuel saving measures and techniques) this is typically in the context of "while this may save fuel, it is very dangerous and no amount of fuel savings is worth a serious injury or your life."

So what are the three? The first is filling my tires (slightly) beyond the recommended maximum. The second is turning off the engine on long downhills. The last and most controversial is drafting trucks. It is this that is the subject of this post.

There are two obvious question. The first is "does it work?" The second is "is it dangerous?" As to its value in fuel savings, while there are some naysayers, most references agree that it is quite effective. Coincidentally, the Discovery Channel hit show Mythbusters covered this topic recently. Their results are summarized here. As is their common practice in such "myths," Kari, Tori, and Grant started with a scale model in a wind tunnel and achieved very encouraging results. Their full scale testing, while it didn't achieve quite the drag reduction of the wind tunnel tests, indicated significant fuel savings even at a following distance as large as 100 feet.

I have played with this idea off and on, both in the Grand Cherokee and in the LR3. However, because I have been so very unsuccessful with the LR3 in achieving the dramatic enhancements to gas mileage I was able to accomplish in the Jeep, I recently decided to pursue this avenue more aggressively. I know that this is controversial, many will call it selfish - if I crash, it will cause a huge backup for those behind me. I agree that this technique is extremely selfish if there is any significant chance that I will crash into a truck and tie up a freeway. I'll get back to that momentarily.

But does it work? According to the information from my Scan Gauge II, it does. I attempted to judge how far I was behind trucks by using my stopwatch to see what fraction of a second elapsed between the back of a truck crossing a highway mark and the front of my car crossing the same mark. Needless to say, tailgating a truck while looking at a Scan Gauge II reading and timing intervals with my stopwatch is sort of living on the edge, but I concluded that I was about 35 feet behind the truck. According to Mythbusters' results, I can look for an increase in miles per gallon of somewhere between 20% and 27%. As best I could tell, I saw an increase from about 21.5 m.p.g. to about 25 m.p.g., an increase of about 16%. These are pretty fuzzy figures - the readouts are constantly changing with changes in slope, etc., and the trucks typically don't maintain a particular speed quite as efficiently as the LR3 (due to the huge mass of the trucks no doubt). Nevertheless, every time I try it I get significant indications of increased fuel efficiency.

So apparently it works, how dangerous is it? Obviously, the worst case scenario is for the truck to apply maximum braking suddenly with no advance indication to me. I don't know if trucks have anti-lock braking systems, for the analysis to follow I won't assume they do. Judging from the condition of a lot of the trucks I see, this is a valid assumption. It should go without saying that, when following a truck at 35 feet, my eyes are focused on the truck's brake lights and my foot is ready to instantly hit the brake pedal. So the question is, when I see his (male pronouns are to be understood as gender-free) brake lights when he implements maximum braking, can I avoid hitting him?

I have to calculate the circumstances under which the distance between my front and his rear decreases to zero. The data necessary for this calculation is his maximum deceleration rate, my maximum deceleration rate, and my time to go from his brake lights on to my application of maximum braking. I will obviously limit my calculations to dry pavement in good condition - I'm not suicidal.

Truck braking, like everything else when looked at by academics, is ridiculously complex, see here if you are skeptical. But the nugget for my purposes in that paper is that the best that an empty semi-tractor trailer can do in deceleration is right at 20 ft/sec^2, or 6.10 m/s^2. For my LR3, it's (maybe, subject to correction) about 8.0 m/s^2. My reaction time, when paying close attention (as I do when tailgating a large truck) was determined using this online reaction timer to be 0.303 seconds in an average of 10 trials. With a lot of practice and knowing what to expect, I was able to bring the average way down, to around 0.20 seconds, but I'll leave it at 0.303 to be conservative.

All right then, we have what we need. The initial conditions are that the truck and my LR3 are each going 60 m.p.h. or 26.8 m/s. The truck hits his brakes maximally and decelerates at 6.10 m/s^2. 0.303 seconds later, I hit my maximum brakes and decelerate at 8.0 m/s^2. How far back must I be to avoid hitting the truck? I suspect that detailed expositions of mathematics bore those who read here (though I'd certainly appreciate any feedback on this). So I'll just state that the answer is that I must be a minimum of 9.67 m or 31.7 feet behind the truck. Now, this assumes I can meet my reaction time and instantly apply maximum braking. I should add, say, a 50% safety factor for a total of 47.6 feet. Call it 50 feet. For those that would say that that's not enough of a safety factor, remember that I assumed that the truck applied perfect braking as well.

While I won't proselytize that drafting trucks is safe and should be done by one and all, I do think that with extreme caution and maximum alertness it can be safely done. I wouldn't want to try to get closer than 50 feet, and I will adjust my procedures accordingly, but I'm keeping this weapon in my arsenal.


halo0 said...

I've been cautiously employing this practice as of late as well, after having basically squeezed all of the mileage increases possible out of the other techniques in my arsenal. One thing to note is that some truckers don't take too kindly to this practice. I've experienced them changing lanes to "shake" me, and just today a truck purposely drove off the road onto the rumble strip. Since this occurred shortly after a rainstorm this action served to throw all kinds of dirt, rocks, and water up onto my car. These are some things to be aware of if you plan on using this technique. I would say though that on average more than half of the trucks I've tried it with don't seem to mind. I usually try to maintain a 2-3 car length distance, which is the closest at which I feel safe at 65 to 70 MPH.

King of the Road said...

I guess, depending on your car, that would be anywhere from 30 to 45 feet. That's in the range of effectiveness, and could be safe by my calculations.
Do you have indications of how effective it's been for you?

halo0 said...

Unfortunately I don't yet own a scanguage, so I can't be too certain. However on my last tank I observed approximately a 4% increase (36 to 37.5 MPG) in my Chevy Cavalier over what I had been getting for the last few tanks. I only drafted on a few occasions, but I suspect the increase was largely due to that, as nothing else changed. After my next fill-up I may have a better idea.

King of the Road said...

If you do get a better idea, I'd be very interested if you'd leave a comment.


Anonymous said...

I draft trucks all the time. I fill up before I leave and fill again when I get there. The difference is very slight after 50 miles of driving unless you are combatting a head wind.

My system is not without error as I fill until it clicks so there might be vapors blocking me adding that extra litre. However the best advice is to simply drive slow

For most cars, you lose 30% once you exceed the posted speed limit

I follow trucks anyways if they're slow and ignore those trucks whizzing by at 70 mph.

Also try to accelerate going down hills and slow doing going up a hill, trucks do this too

Anonymous said...

You are an idiot if you think you are being "safe" by depending on a 0.3 or 0.5 sec reaction time.

King of the Road said...

Well "anonymous," an ad hominem attack isn't very productive. Explain why my analysis is incorrect and I'd be happy to look at what you write and respond. If I'm wrong, I'll say so. But considering I draft with my foot literally hanging above the brake pedal and my attention focused on the truck ahead, I don't know why the timer results wouldn't be applicable. I routinely achieve under 0.2 seconds when concentrating.

Brian said...

If you are already in contact with the semi when he brakes, then no matter how hard he brakes, your car will brake at the same speed and suffer no damage. Of course this could have some implications in being able to control the car during high decelerations. Just a thought.

Anonymous said...

I would think that you would be very attentive while doing this at first. I would also think you would become more relaxed over time and therefore not be as quick on the break as you would just starting out. This could lend itself to hypmotism slowing your reaction time. I would think at that point your math would have to be recalculated.

Anonymous said...

I recently came across your blog and have been reading along. I thought I would leave my first comment. I dont know what to say except that I have enjoyed reading. Nice blog. I will keep visiting this blog very often.



Unknown said...

Austin, TX to Destin, FL and back on 3-people + bags loaded Tiguan: global average MPG 27.4; highway at 65 mph: 32–34 mpg vs. 36-37 mpg if following a truck behind (each measurement type was averaged over ~15 miles three times), at ~2× my car’s lengths (tailgating or “drafting” or “hypermiling” behind a truck in the “free ride zone”). Best-case-scenario estimate: 37 – 32 = 5 mph or 5/32 = 16% increase in mpg. 32 mpg = 7.35 L/100 km = 3.125 gal/100 miles; 37 mpg = 6.36 L/100 km = 2.704 gal/100 miles, thus on 750 miles × 2 = 1,500 miles (2,414 km) round-trip, (3.125 – 2.704) = 0.421 gal/100 miles saving × 1.5 = 0.632 gal or × $3/gal = $1.89 saved. Without head wind = NOT WORTH IT. asemyonov@gmail.com

Carl@TeslaTouring.com said...

It appears that many comments are from drivers that do not have Traffic Aware Cruise (Tesla's term) which will stop your car to a complete stop with the car you are following at the number of car lengths you set from 1 to 7 (14.7 ft. = one car length).