There is an area of relative low pressure behind a car that “sucks” the car backwards. This is one component of the force of drag acting on the car. When the front car is being drafted, the pressure of the air behind it increases and drag is reduced. Both cars benefit.
This is why in racing you regularly see cars drafting one another. Collectively, they can go faster than one car alone.
Yes, and since this is an old segment rerun, Wolfgang will come on next week and give the correct answer.
OOPS
But you get bonus points for knowing the right answer. 
"both cars benefit"
Until one in front has to make a sudden stop. So, it has little practical safe use on today’s highway…but a good old discussion.
Bicyclists experience the same benefits.
As an off grid homemade power geek, I found the ‘drafting’ discussion to have some similar issues to a situation that has intrigued me for years.I had heard that one could ‘harvest’ leaked energy from high voltage power lines by deploying an ‘antenna’ wire below them and feeding the resulting D.C. voltage to storage batteries or some load. I actually did a field check on the concept while visiting an off grid community in Baja … the barbwire fence under the transmission lined was ‘downloading’ about 7 volts per 200’. But I was shocked (shocked I say!) to read recently that the power companies would be able to sense this, shall we say ‘power drafting’ and come after you for theft, how disappointing! Apparently in this case availing oneself of the seemingly free wastage causes a measurable increase in resistance in the line… leading me to believe there is no such thing as a free lunch!
A way to think about it is to just consider 2 areas of drag on a vehicle. the frontal shape and cross section hits resistance in the air, and the tail (or base) if not ending in an “ecomodder’s” low drag tail produces non-slipstream eddy air flow. This is a low pressure zone that acts like wanting to pull the car backwards. When 2 cars like this pull up close and closer, it’s not as if the front “pulls” the rear car increasing it’s fuel economy, but rather that slipstream concatenates to the air flow of the rear car reducing the backward pull on the front car and reducing the frontal air resistance of the 2nd car. They travel just as if they were just one longer vehicle now composed of just one frontal resistance and rear resistance to contend with.
Timely of interest was this week’s nhk’s scienceview “wind lens” for wind turbines and the discussion of using eddy currents to decrease air pressure and increase wind flow and thereby turbine output.
Guess Tom and Ray are not motorsports fans. Not only is drafting common on high speed tracks, but even more revealing is “slingshotting.” This move utilizes the low pressure area behind vehicle 1 to allow #2 to accelerate, use that speed advantage to pull around #1, while the first one is slowed by loss of that trailing vehicle.
Often used for not just cars, but motorcycles as well.
At my age, shingshotting, drafting, all scare me. I have trouble drafting couples ahead of me while doing the Texas Two Step.
"both cars benefit"
Until one in front has to make a sudden stop
+1
And, I can add that, if somebody wants to “draft” my car, they won’t be doing it for very long, because as soon as I see somebody who is tailgating extremely closely, I take my foot off the gas until they decide to pass me.
Just because somebody is stupid enough to try this maneuver, that doesn’t mean that I have to permit them to do it.
I remember a test in one of the car magazines many years ago that was conducted at TRC 7.5 mile oval in Ohio. They ran cars at top speed to see what the top seed was and how the cars behaved. Two of the cars were a Porsche 928 S4 and a Corvette. The 928 had a top speed of 170+ MPH and was the faster of the two. When they tried drafting with the Corvette behind the 928, the 928 picked up some speed and the Corvette was able to stay behind it. This would certainly seem to indicate that the front car does benefit as the rear car would smooth out the airflow behind it where a lot of the drag is created.
Dont know about this discussion I can draft way back with my shoebox aero Dakota,was drafting a boxy Toyota SUV up a grade called “North Mtn” and believe me you I wasnt anywhere close to being close to this vehicle from a safety standpoint,this driver seen what I was doing ,so the driver hit the hammer lane and didnt even speedup,so after a quarter mile or so of His hogging the left lane ,I floorboarded it and went by him on the right,His actions bugged me because believe or not I was probaly 5 truck lengths back and it was probaly helping him as much as me,but in retrospect the catabatic flow down the slope was probaly what this persons vehicle was breaking up,in another words that driver was probaly breaking the headwind.So I firmly believe drafting helps,but everyone doesnt appreciate(especially when you would happen to be riding the others bumper
-Kevin
Here is a simulation of drafting,
In the real world things work out slightly differently. My family used to rent two identical cars every year to drive from Salt Lake City to visit various family members throughout Idaho. Because my father and I are both engineers, we kept meticulous records of gas consumption and we noticed that the car in front, regardless of the driver or terrain, consistently got better gas mileage. Our theory was that it was difficult for the 2nd car to stay close enough to the front car to benefit from the aerodynamic advantage and consequently it was difficult for the driver of the rear car to avoid occasionally tapping the brakes or hitting the gas pedal to maintain the proper distance. We repeated the same experiment over several years with various different makes and models of cars. We always had the same result.
This might make a good puzzler.
Turbulence is, apparently, very unpredictable. I’ve been trying to figure out the aerodynamics of hauling and 8.5 ft. wide house on a trailer. airtabs.com shows some research from NASA and has an interesting discussion about drafting and the gaps between vehicle and trailer when hauling. There are turbulent issues between those closely-linked vehicles. Drafting would increase that distance.
When two NASCAR race cars draft together both cars go noticeably faster, so common sense would tell you that both cars benefit from running nose-to-tail.
True story - one day I was driving my semi down the highway at 60 mph, and all of a sudden my truck noticeably sped up and I had to back off the gas in order to continue to maintain 60 mph. I couldn’t figure out what was going on, so I checked my mirrors and sure enough the semi that had been following me had pulled up closer and was now glued to my trailer’s rear bumper, and that made both trucks more efficient and caused my truck to speed up.
@Tony Carlos: Shake ‘n’ BAKE!
@McCormack: Doesn’t this work in Nascar because of “bump drafting?” Nice theory but hard to apply in the real world.
I was trying to think of an instance where being drafted would be an aero detriment and I think I found one:
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a small vehicle tailed by a much larger one. The lead vehicle would have at least a partial boattail, so that it produces a very small wake normally.
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Because they are moving at subsonic speeds, the air can “sense” the large obstruction of the large vehicle close behind. Thus, as the air moves out of the way of the obstruction, it creates flow seperation on the boattail, increasing the size of the wake.
So a Prius being tailgated scary-close by a cabover semi would likely lose MPG…
I know that this is an old show, but it is an older question. All I know is that in my younger days…1950s, '60s, '70s…long before the C.D.L. was invented, and back in the day when there was a long-haul truck driver’s “fraternity” of sorts, and before so many of the drivers were quite so impolite, incompetent, or illiterate in English…the long-haul drivers would form “convoys” of up to six to ten tractor-trailers and run for many hours on end at a distance between them of only about three to eight feet! In the mid-west at least they claimed that this practice saved about one to one and a half miles per gallon, and when one is using six miles per gallon, that is a LOT! Now I wouldn’t ever suggest this, but it does work. And probably requires a lot of amphetamines.
A good way to experience the turbulence that exists behind vehicles is to ride a motorcycle on the highway. You’d be amazed at how turbulent it is behind some trucks. Eighteen wheelers are bad as expected, but many SUVs are equally turbulent. And some which you would guess are bad turn out to not be so.