Aerodynamic Drag Explained

Watch the video to learn how aerodynamic drag is measured, what the standards are,
its effects on fuel economy, and how all of this determines your return on investment.

Drag Effects Mileage

At highway speed, aerodynamic drag accounts for more than half of your fleet’s fuel consumption.

Reducing drag is the quickest and most cost effective way to reduce your fleet’s fuel consumption and carbon footprint.

A typical tractor pulling a bare dry van creates about 65 square feet of drag. The effectiveness of aerodynamic devices is now measured in terms of how much of this drag area they remove.

The GHG2 standard established by the EPA requires devices to be tested to a uniform standard of drag reduction. They are then classified on the amount of drag area (expressed in square meters) they reduce.

Once the baseline drag and drag reduction amounts are known, fuel savings can be calculated.

Delta CdA Matters

For semi-trailers, the EPA uses a number called Delta CdA, which is a technical way to express how much of the drag area that an aero device removes.

For example, if a device is has a .40 Delta CdA, it removes .40 square meters of drag area from the tractor-trailer combination. The device is classified in a bin according to the amount of area it removes. The bins help account for the uncertainties inherent in testing (1).

While at first confusing, the new standard makes evaluating and comparing aero performance easier. There are now only two practical test methods (CFD and Wind Tunnel). After literally hundreds of test runs using both methods, we came to the same conclusion that the government researchers did. While neither method is perfect for every device, they generally yield very similar results.

We have also done hundreds of road tests, covering thousands of miles, on both tracks and actual highway conditions. Again, our results generally agree with the government researchers: with some important exceptions.

(1) NHTSA GHG Phase 2 Trailer Implementation Worksheet, November 16, 2016

Delta to Dollars

Annual Savings at Various Fuel Costs
Savings Over Years

Our model is based on the 2015 Canadian NRC Report: “Improving the Aerodynamic Efficiency of Heavy Duty Vehicles”, 2015, p.19. These numbers are only a guide. You should derive your own conclusions based on your specific fleet’s data.