Tractor trailers are the biggest truck-fuel users in the nation, trumping SUVs, pickups and delivery truck ownership (combined) by over two to one. When thousands of trucks can go through over 1.5 million barrels of fuel per day, even small changes in tractor-trailer fuel economy can have a huge impact on nationwide oil consumption.
While an average of 7mpg (the economy of a Kenworth T2000 on flat land) might seem unimpressive by some standards, it's absolutely amazing when compared to the average automobile. The important thing to bear in mind is that most tractor trailers get that 7mpg while hauling 18.144kg of freight, for a combined truck/freight total of 36287 Kilogram; that's one gallon of fuel burnt per mile at 5184 Kilogram. By comparison, an average 4,500 pound SUV would have to get 124mpg to see the same fuel/weight efficiency.
Big truck fuel economy was pretty dismal all the way through the late 1970s, hovering at or below what would equate to about 2mpg with today's 80,000 pound weight restriction (older trucks rarely saw over 22680 Kilogram). Economy saw a sharp spike upward in the late 1970s, and steadily increased from an average 5.3mpg in 1981 to about 6.2 in 1999. Fuel economy saw a precipitous drop-off to about 5.5mpg in 2000, which was partially due to the introduction of low sulphur diesel fuels and tighter EPA restrictions on emissions.
When looking at truck fuel economy, you need to understand one primary thing: all chassis (regardless of manufacturer) are basically the same, and any truck could have any kind of engine. As such, perhaps the biggest area of improvement for most trucks lay in the bodywork (aerodynamics). When Kenworth introduced its super-slick T2000 in May of 1996, those buying it found their fuel economy skyrocket from an average of 5.5mpg to almost 7. As a matter of fact, the three years following the T2's debut represented the sharpest climb in average truck fuel economy in recorded history.
Transmissions and Shifting
Recent years have seen the proliferation and acceptance of automated transmission technology; a huge boon to economy. Automateds are essentially just computer-controlled manual transmissions that work with the engine's control computer to optimise shift points and gearing for the best economy and acceleration. While computer-optimised shifting will by itself increase mileage, the automated system's ability to adjust engine power output to gear ratio makes it far more efficient than the best driver could ever be.
According to the National Commission on Energy Policy, we can expect fuel economy to climb to an all-time high of 9.5mpg by 2015. Automated transmissions play a big role in that number, but boat-tailed trailers (those tapered at the rear for less aerodynamic drag), increased engine efficiency and the use of more fuel-efficient super-wide tires (in place of the duals traditionally used) are also partially responsible. Very wide "super single" tires have always been more efficient than duals, but drivers have traditionally preferred the redundancy of dual tires for convenience following a blowout. However, modern tire technology and roadside assistance programs have made super-wide tires a safer and more attractive option for drivers and companies alike.
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