question: How close do you have to be to a car to benefit from "drafting". Can you get better gas mileage while still keeping a good distance?
comment: It depends on the aerodynamics of the cars involved. Pull your minivan a couple car lengths behind a semi-trailer, you'll feel the drafting effect. If you have two fast cars with good aerodynamic coefficients, you need to be less than a foot apart.
passages: Drafting was discovered by stock car racers in the 1960 Daytona 500, when Junior Johnson found that he could use drafting as a strategy that helped him overcome the fact that his Chevrolet could not keep up with other cars, allowing him to win the race. Like Johnson, other drivers found they picked up speed running closely behind other cars; and as they experimented they found that a line of cars could sustain higher speeds and/or use less gas (resulting in fewer pit-stops) than a single car running by itself.
   Some drivers have been known to draft behind other vehicles, particularly tailgating larger vehicles, to save fuel. For example, hypermilers using this technique can achieve 75 mpg or more (a 10% increase in efficiency of certain hybrid vehicles). Some sources say that the most common tailgating does not save gasoline even at freeway speeds because one is likely to accelerate and brake so frequently that any aerodynamic savings are lost through the brakes.
   On the show "MythBusters", drafting behind an 18-wheeler truck was tested and results showed that traveling behind the truck increased overall mpg efficiency by 11%. Traveling behind the truck produced a 39% gain in efficiency. Additionally, on the same episode, Mythbusters demonstrated that it can be very dangerous for the following car if one of the truck's tires (or their recaps) delaminate, as the chunks of ejected rubber can be large enough to cause serious harm, even death, to a driver following too closely.
   Drafting (aerodynamics)
  
  Drafting or slipstreaming is an aerodynamic technique where two vehicles or other moving objects are caused to align in a close group reducing the overall effect of drag due to exploiting the lead object's slipstream. Especially when high speeds are involved, as in motor racing and cycling, drafting can significantly reduce the paceline's average energy expenditure required to maintain a certain speed and can also slightly reduce the energy expenditure of the lead vehicle or object.
  
  Section::::Drafting in sport.
   Drafting is used to reduce wind resistance and is seen most commonly in bicycle racing, motorcycle racing, car racing, and speedskating, though drafting is occasionally used even in cross-country skiing, downhill skateboarding, and running. Some forms of triathlon allow drafting. Drafting occurs in swimming as well: both in open-water races (occurring in natural bodies of water) and in traditional races in competition pools. In a competition pool a swimmer may hug the lane line that separates them from the swimmer they are abaft of thereby taking advantage of the liquid slipstream in the other swimmer's wake. Drafting also occurs in competitive longboarding.
   Drafting is most important at NASCAR's restrictor plate tracks, Talladega Superspeedway and Daytona International Speedway, where the plates mean that much less power is available to push the large bodies through the air. Race cars reach their highest speeds on these superspeedways, so the aerodynamic forces are highest, and the effects of drafting are strongest. Since restrictor plates were first used as a safety device, their effect has changed the nature of drafting. Vehicles no longer have sufficient horsepower or throttle response to maintain their drafting speeds upon exiting the draft; they can pull out and squeeze ahead but lack the response to clear the car being passed. This negates the slingshot maneuver. As a result, passing is often the result of cooperation between two or more drivers or is achieved by sucking air off the side of the car being passed, a technique called side-drafting.
   Kyle Busch is largely responsible for a different type of bump drafting, which is now referred to as "two-car drafting" and "tandem drafting". At a 2007 test session in Talladega, he asked Ryan Newman to push him from behind, and was stunned to realize he was two seconds faster with Newman's help. At the newly paved Daytona International Speedway in 2011, Busch was the first to realize that the corners were smooth enough to allow a two-car draft for the complete length of the track. During test sessions on the track, when Busch was pushed by his brother Kurt's Penske Racing teammate Brad Keselowski, they ran 15 mph faster than single cars. Other drivers quickly picked up on Busch's strategy, and the two-car draft dominated the 2011 Daytona 500 and Budweiser Shootout. This strategy had also been very prominent at Talladega. In 2011, two-car tandem drafting was used for the extent of the Aaron's 499, with many drivers drafting their own teammates (e.g., Jimmie Johnson and Dale Earnhardt, Jr. drafted together, as did Jeff Gordon and Mark Martin). For the 2012 season, the Sprint Cup series cars were modified in a way that made the tandem impossible, in order to return to pack racing. In 2014, bump drafting was banned by NASCAR in the Nationwide Series and Camping World Truck Series.
   Drafters also face the danger that, if the vehicle in front stops suddenly, there is little time to react. Platooning is under study as a means to get the benefit while also increasing safety.
  
  Section::::Computer simulation of drafting.
   Bump drafting is a tactic used at Talladega and Daytona. The technique was initially popularized by the Archer Brothers in the SCCA Sportruck series during the late 1980s. It begins as normal drafting, but the following car pulls up behind the lead car and bumps into the rear of it, pushing the lead car ahead, to maintain momentum.
   Section::::Drafting in sport.:Running.
  
  Drafting behind another runner can conserve energy, although the effect is less than in cycling due to the fact that speeds are lower. 
   Similar to the "Belgian tourniquet" in cycling, the "slingshot pass" is the most dramatic and widely noted maneuver associated with drafting. A trailing car (perhaps pushed by a line of drafting cars) uses the lead car's wake to pull up with maximum momentum at the end of a straightaway, enters a turn high, and turns down across the lead car's wake. The combination of running downhill and running across the zone of lowest aerodynamic drag allows the trailing car to carry extra speed and pass on the inside of the leader.
   Section::::Techniques.:Drafting.
  
  "Drafting" occurs where a smaller vehicle drives (or coasts) close behind a vehicle ahead of it so that it is shielded from wind. Aside from being illegal in many jurisdictions it is often dangerous. Scale-model wind tunnel and Real-World tests of a car ten feet behind a semi-truck showed a reduction of over 90% for the wind force (aerodynamic drag). The gain in efficiency is reported to be 20–40%.
  
  Section::::Techniques.:Energy losses.
   In recent years, as aerodynamics have become increasingly critical to the performance of stock cars on "intermediate" oval tracks (between 1.33 and 2 mi) and superspeedways not requiring restrictor plates, the effect of turbulent, or "dirty" air when following closely behind another car has become much more akin to that described above in open-wheel racing (a situation described in NASCAR circles as "aero push"), and is often cited as a main reason for a decrease in the amount of overtakes. 
  
  Section::::Drafting in sport.:Motorsport.:Stock car racing.:Bump drafting.
   If done roughly or in the wrong position (e.g. close to the entry of the turn), this tactic can destabilize the handling of the lead car sometimes causing a crash. Use of the tactic in this manner is known as "slam drafting". Due to the danger, NASCAR has attempted to limit the bracing on bumpers on cars, disallowed bump drafting in turns, introduced "no bump zones" on certain portions of speedways where this practice is prevalent, and penalized drivers who are too rough in bump drafting. The 2010 NASCAR season allows drivers more freedom. Bump drafting is allowed anywhere, including turns.
   Section::::Drafting in nature.
  
  Animals have been observed to use true drafting behavior reminiscent of auto racing or cycling. Caribbean spiny lobsters for example are known to migrate in close single-file formation "lobster trains".
  
  Section::::Vortex surfing.
   Handling in corners is affected by balance changes caused by the draft: the leading car has normal front downforce but less rear downforce. The trailing car has less front downforce but normal rear downforce. A car with drafting partners both ahead and behind will lose downforce at both ends.
   BULLET::::- Following in the slipstream of another motor vehicle, or "drafting", allows for significantly improved fuel efficiency due to reduced atmospheric drag. Truck convoys are a common example, travelling highways in a single-file queue several vehicles long. In tests, this has been shown to produce significant fuel savings. Auto racing drivers also draft in order to conserve fuel, the better to gain competitive advantage by reducing the frequency of fuel stops or, more often, to reach a higher speed before pulling out to attempt to overtake another driver.
   Computer simulation (computational fluid dynamics or CFD) is increasingly being used to analyse drafting. It is important to understand the aerodynamic behaviour of a motor vehicle when drafting, for example if the rear car is too close to the front car, the air supply to its radiator will be reduced and there is a possibility of the engine overheating. Most motor sport aerodynamic analysis is performed using wind tunnel testing. This becomes difficult for drafting cases, if only because a very large wind tunnel is needed. CFD, a kind of virtual wind tunnel, is used by race teams to understand the car's performance while drafting.
   Drafting
  
  Drafting or draughting may refer to:
  
  BULLET::::- Campdrafting, an Australian equestrian sport
  
  BULLET::::- Drafting (aerodynamics), slipstreaming
  
  BULLET::::- Drafting (writing), writing something that is likely to be amended
  
  BULLET::::- Technical drawing, the act and discipline of composing diagrams that communicates how something functions or is to be constructed. E.g.:
  
  BULLET::::- Architectural drawing
  
  BULLET::::- Electrical drawing
  
  BULLET::::- Engineering drawing
  
  BULLET::::- Plumbing drawing
  
  BULLET::::- Structural drawing
  
  BULLET::::- Textile manufacturing weaving pattern
  
  BULLET::::- Conscription into compulsory military service
  
  BULLET::::- Draft (sports), where new players are chosen by the teams rather than the players choosing their teams
   When cyclists ride fast they form a "paceline". Each cyclist, except the first, is drafting behind another one. In order to ride very fast, a team of some skilled cyclists may form the "Belgian tourniquet". Successively, each cyclist leads the group. Drafting can be "cooperative": several competitors take turns in the lead position (which requires the most effort and energy consumption). It can also be "competitive" or "tactical": one competitor will try to stay closely behind another, leaving him or her more energy for a break-away push to the finish line.
  
  Section::::Drafting in sport.:Motorsport.
  
  Section::::Drafting in sport.:Motorsport.:Road racing.
   In single seater, open wheel racing series such as Formula One and the IndyCar Series, as well as to a lesser extent in sports car racing, a technique known as "slipstreaming" is used. Along a long straight a car following close behind another uses the slipstream created by the lead car to close the gap between them, hoping to be able to overtake the leader under braking for the next corner, or if he has a straightline speed advantage, to pass on the straight. However it is very difficult for cars to follow each other close together in fast corners as the "dirty" (turbulent) air that comes off the lead car unbalances the trailing car as its aerodynamic devices provide less grip. 4-time Indianapolis 500 winner Rick Mears was quoted by motorsport.com writer David Malsher in 2015 as saying "The fact is, grip created by aerodynamic downforce is our enemy… At the moment, the downforce is so great that it masks the handling and hurts the racing." 
   Section::::Techniques.:Safety.
  
  There is sometimes a tradeoff between saving fuel and preventing crashes.
  
  In the US, the speed at which fuel efficiency is maximized often lies below the speed limit, typically ; however, traffic flow is often faster than this. The speed differential between cars raises the risk of collision.
  
  Drafting increases risk of collision when there is a separation of fewer than three seconds from the preceding vehicle.
   It is believed, but not yet conclusively proven, that Thoroughbred racing horses draft each other, especially in longer races.
  
  Section::::Drafting in sport.:Cycling.
  
  In cycling, any time one bicyclist is riding behind another, energy is conserved, especially at higher speeds.
  
  In road bicycle racing, the main (largest) group of tightly packed cyclists in a race is called a "peloton" where cyclists ride in a long formation with each (but not the first rider) drafting behind the others before them.
   Section::::Drafting in sport.:Motorsport.:Stock car racing.
  
  On the faster speedways and superspeedways used by NASCAR, and ARCA two or more vehicles can race faster when lined up front-to-rear than a single car can race alone. The low-pressure wake behind a group's leading car reduces the aerodynamic resistance on the front of the trailing car allowing the second car to pull closer. As the second car nears the first it pushes high-pressure air forward so less fast-moving air hits the lead car's spoiler. The result is less drag for both cars, allowing faster speeds.
   Nike worked with the aerodynamics expert Robby Ketchell at the University of New Hampshire to experiment with and select a formation of pacemakers that would best minimize drag on the professionals it sponsored in the project it termed Breaking2. A Wired magazine report that interviewed various experts affiliated and unaffiliated with Nike found they universally expected more coordinated pacing efforts to occur in running after Breaking2, with two of the quoted experts predicting that behavior like "cooperative drafting," or races that incentivize cycling-peleton-like behavior could improve running times.
  
  Section::::Tailgating and hypermiling.
  