Car Vs. Tractor-trailer

by Richard Rowe

Tractor-trailers and cars are designed with two very different philosophies in mind. Cars, by and large, are designed to compete with each other within a segment. It's all about "which SUV gets the best gas mileage," "which sedan is the safest" and "which sports car is the most powerful." Truck designers are very limited in terms of design flexibility. The truck has to pull 80,000 pounds, have certain exterior dimensions and use certain types of tires and engines.

Specs.

The average passenger car is a front-wheel drive midsize or compact weighing about 3,250 pounds, with an average fuel economy of 27.5 miles per gallon. The average tractor-trailer is just over 80 feet long, 13 feet, 6 inches high, and about 8 feet wide. Fully loaded weight for most comes in at 80,000 lbs. (federally mandated); weight with an empty trailer can vary between 30,000 and 45,000 lbs. A bobtail (just the truck with no trailer) weighs between 15,000 and 20,000 lbs.

Power

The average passenger car engine is a fuel-injected four-cylinder about 180 cubic inches in displacement, producing about 200 horsepower. The average transmission is a four-speed automatic, but options vary greatly from direct drive (no transmission) for electric cars to continuously variable transmission (CVTs) with no end to the possible gear ratios. Most trucks use 16.0-liter (976-cubic-inch) six-cylinder turbodiesels producing between 350 and 550 horsepower and use a 10-speed manual or computer-controlled manual transmission.

Acceleration and Stopping

The average family car accelerates from zero to 60 mph in about eight seconds and can decelerate from that speed within about 140 feet, but some sports cars (like the Nissan GT-R) can accelerate to 60 in under 3.5 seconds and make it back to a standstill at around 100 feet. Compared to such cars, even a really powerful 550 horsepower tractor-trailer is a slug: 0-60 in 35 seconds when fully loaded and 20 seconds when empty. However, that same truck can go from zero to 60 in just over 10 seconds without a trailer, which is comparable to many street cars. Stopping distances for a fully loaded truck traveling 60 mph average a dismal 400-plus feet and can actually be worse with the weight removed.

Fuel Economy

Big trucks are amazingly efficient for their size. Although 6 miles per gallon (fully loaded at 80,000 pounds) may not seem like much, you have to bear in mind that fuel economy drops linearly with weight. An average 3,250-pound passenger car would have to average about 147 mpg to match the weight/fuel economy ratio of a 6 mpg truck.

Safety

Where safety's concerned, trucks are becoming more car-like every year. Not long ago, the anti-lock brakes and a single driver's side airbag were the truck's only additional safety system beyond its inherently redundant air-braking system. Now however, many new trucks use the same stability control and traction control systems as cars. Mercedes-spec intelligent cruise control and braking systems (which uses radar to detect cars ahead and behind, allowing the computer to automatically modulate speed to maintain a safe distance) are still a few years off for trucks, but will almost certainly make it to big rigs once the technology proves itself reliable.

About the Author

Richard Rowe has been writing professionally since 2007, specializing in automotive topics. He has worked as a tractor-trailer driver and mechanic, a rigger at a fire engine factory and as a race-car driver and builder. Rowe studied engineering, philosophy and American literature at Central Florida Community College.

Photo Credits

  • photo_camera line of trucks 1 image by Aaron Kohr from Fotolia.com