Flaps

• The skirt is not just for mud.truck image by max blain from Fotolia.com
  
  The flaps underneath the semi truck trailer are actually there to improve performance. According to Greencarcongress.com, "Wind skirts under the trailer significantly reduce wind resistance and reduce airflow around the trailer. This is a big fuel economy benefit."
  
  

Fuel

• The fuel additive acetone increases mileage.gas station image by Mat Hayward from Fotolia.com
  
  Studies have shown that adding acetone to the fuel of semi trucks can improve performance. In fact, according to Pureenergysystems.com, adding acetone at 2 percent of volume may increase the gas mileage by as much as 4 to 7 percent.
  
  

Increase in Use

• Expect more trucks on the road.Cement trucks 3 image by beverly from Fotolia.com
  
  According to the Department of Transportation, freight tonnage is expected to increase 70 percent between 1998 and 2020.
  
  

Length of Trailer

• The turning radius is affected by pin placement.Cargo truck image by Canakris from Fotolia.com
  
  The average semi truck trailer is between 28 feet and 57 feet. The turning radius of these trucks is affected by the position of the kingpin. This is the pin that connects the trailer to the truck. According to the Federal Highway Commission, "the longer the kingpin setting, the larger the turning radius."
  
  

Brake Time

• The distance needed to stop is much greater than that of an automobile.American container truck on road of my trucks series image by alma_sacra from Fotolia.com
  
  The average stopping time of an 18-wheeled semi truck is 40 percent higher than that of a car. That means if the truck needs to brake suddenly, it needs 40 percent more distance than the average automobile.
  
  

Weight

• According to Thetruckersreport.com, the maximum legal weight for an 18-wheeled truck is 80,000 pounds, or 40 tons. For comparison, the average personal vehicle weighs around 5,000 pounds. This is one reason why many city roads do not allow large trucks.