SAE Tested

The Joint TMC/SAE Fuel Consumption Test Procedure Type II – often called SAE J1321 testing, a reference to its document number in the Society of Automotive Engineers library – is the industry standard for measuring increases in fuel efficiency attributable to aerodynamic technologies. 


SAE J1321 testing involves a control vehicle and a test vehicle. The control vehicle and test vehicle are identical, featuring the same engine, tractor, trailer and cargo, except that the test vehicle is equipped with the technology being measured for fuel efficiency. 

The control vehicle and test vehicle are operated by trained drivers and are driven in tandem through the same drive cycle, or run. A single SAE J1321 test consists of several runs. Each run may be 60+ miles, consisting of multiple laps around a test track. 

At the beginning and end of each run, the vehicles' detachable auxiliary fuel tanks  are weighed using the gravimetric method in order to determine how much each vehicle consumed over the course of the run. 

After several runs, fuel consumption data can typically demonstrate a statistically significant relationship between fuel efficiency and the equipment being tested. SAE J1321 testing calls for the ratio between the fuel consumption of the control vehicle and of the test vehicle, or the T:C ratio, to be within 2 percent. 

Other testing methods do exist. For example, wind-tunnel testing and computational fluid dynamic modeling can be used to calculate reductions in the drag coefficient (Cd) achieved by aerodynamic technologies in a controlled laboratory setting. On the other end of the spectrum, trucking fleets may conduct trials of new products along their everyday routes, complete with traffic and crosswinds. 

While each of these methods has a role in research, development and commercialization, laboratory testing and fleet testing are no substitute for the level playing field of SAE J1321 testing. Laboratories employ an inconsistent array of hardware and software. Trucking fleets may have unique routes that make the results of fleet testing applicable only to them. 


Smartway Transport Partner ATDynamics

For all of its constraints, SAE J1321 testing still allows for variability that may limit the applicability of test results. For example, if the runs are conducted on a very windy day, then technologies that increase fuel efficiency by mitigating crosswinds, such as side skirts, may achieve particularly strong test results. But those results may not be indicative of the performance of the technologies in non-windy conditions. 

To ensure the broad applicability of test results, the U.S. Environmental Protection Agency SmartWay Transport Partnership (SmartWay), has adopted a modified version of SAE J1321 testing that sets limits for wind speed, temperature, test track layout, altitude, precipitation and other variables. 

This test method now provides the basis for aerodynamic devices to qualify as U.S. EPA Verified Technologies. 

ATDynamics TrailerTail® and Skirts are U.S. EPA Verified Aerodynamic Technologies in addition to having gone through dozens of SAE J1321, wind tunnel, CFD and on-road tests with researchers and trucking fleet customers.

To further discuss testing data and methods, please contact

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