"Now that fuel is of greater concern to drivers and companies, here are some rules of thumb your readers might find interesting.
"From a Goodyear white paper... "'Cut down top speed. Each 1 mph over 55 costs you 2.2% in fuel costs!'"
"From a Kenworth white paper... "'Approximately half the energy used by a truck traveling 55 mph is to simply move the air around that truck. At 65 mph, about two-thirds of the energy is used to cut through the air.'
"From a Cat white paper... "'Looking at the 80,000-pound tractor-trailer, when speed increases from 65 to 70 MPH, the engine must develop an additional 45 horsepower to meet the Total Wheel Demand horsepower. This is a significant 18 percent increase! Aerodynamic Resistance horsepower accounts for 32 horsepower and represents 71 percent of the increase.'"
"'As a rule of thumb, fuel consumption increases approximately 0.1 mpg for every 1 mph above 55 mph, assuming that the vehicle is properly spec'd and the transmission is in top gear. In other words, increasing vehicle speed from 65 mph to 70 mph increases fuel consumption and reduces the fuel mileage by 0.5 mpg.' [The Technology & Maintenance Council of ATA has long preached the same rule.]
"Now here is the most interesting fact. As the truck OEMs develop more aerodynamic tractors, the trailer now becomes the critical part of reducing overall aero drag for the tractor-trailer unit. Here is statement from a NASA report:
"'Because base drag (the back of the trailer) increases as forebody drag (the tractor) is reduced and these components of drag are additive, afterbody (the trailer) refinement (base drag reduction) will be required in order to achieve an overall aerodynamic drag coefficient of 0.25.'"
"The above statement is demonstrated by [owner-operator] Gina Angsten's comments from her blog when she had the opportunity to pull an aero-equipped trailer with her new Cascadia:
"'I thought I would update you all on my fuel mileage. I was able to pull one of my company's new trailers. The company I pull for, which is Duplainville Transport, is a Smartway company and we just got in a bunch of new trailers. I pulled one for a week and was able to get 7.5 MPG pulling 43,000-pound loads and running 62-63 mph. My route was Wisconsin to Georgia to West Virginia and back to Wisconsin.
"'Along with the Cascadia having the aerodynamics and having super singles and the Airtabs, all play a part in my fuel mileage. Put all of that with a trailer that also has super singles, Airtabs, gap reducers and skirts on the trailer. It made all the difference in the world for fuel mileage.'"
The aero trailer's difference is a gain of 0.7 mpg, based on Gina's average of 6.8 mpg over the previous 100,000 miles. Her blog is here. Duplainville Transport's aero trailers were featured in HDT's November 2010 issue.