March 15, 2017 4:00 PM, EDT
Spec Aero Devices With Fuel Calculator

This story appears in the March 13 print edition of Equipment & Maintenance Update, a supplement to Transport Topics.

All fleets want to improve fuel economy, and many turn to aerodynamic devices to attain their fuel-consumption goals, but there’s a lot of trial and error that takes place in the process. Wouldn’t it be nice to know in advance what to expect from aerodynamic supplier offerings?

Marc Clark

Over the past few years, manufacturers have introduced a wide range of aerodynamic devices claiming varying degrees of fuel-economy savings. The U.S Environmental Protection Agency’s SmartWay program has established some guidelines to help evaluate these claims, but ultimately, total actual savings vary dramatically based on a fleet’s typical drive cycle.

Many fleets have several divisions with many drive cycles, making the analysis even more difficult. As a result, equipment owners often do nothing to make aerodynamic improvements, or they specify devices based on guesswork or limited data.

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That’s unfortunate, because aerodynamics are unquestionably important to achieving fuel-economy gains. Based on U.S. Department of Energy research, fuel economy is notably affected by aerodynamics at speeds of more than 35 mph. At 65 mph, about 53% of the horsepower generated by a combination vehicle is used just to move the unit through the air.

Add to this the dynamic and complex nature of crosswinds, which are a significant factor in calculating fuel-economy savings of aerodynamic devices. What’s more, results for individual aerodynamic devices cannot be simply added together to obtain a realistic result. Combinations of devices have to be tested together to properly evaluate the combinations’ overall benefit, which further complicates the task at hand.

TMC currently offers three methodologies for testing fuel economy:

• RP 1102A, TMC In-Service Fuel Consumption Test Procedure — Type II. This RP provides a standardized test procedure for comparing the fuel consumption of two conditions of a single test vehicle or of one test vehicle to another when it is not possible to run the two or more test vehicles simultaneously.

• RP 1103A, TMC In-Service Fuel Consumption Test Procedure — Type III. This RP provides a standard test procedure for comparing the fuel economy of components or systems of the type that cannot be switched from one vehicle to another in a short time. This test procedure also is ideally suited to comparing the fuel consumption of one vehicle to another, and one component of a combination vehicle to the other vehicle without the component in another.

• RP 1109B, Type IV Fuel Economy Test Procedure. This RP provides a test procedure for comparing the fuel consumption of two vehicles of similar capabilities or one unit of a combination vehicle to the same unit of another combination vehicle. This procedure also provides for evaluation of the effects of certain components or systems on fuel economy. This version permits valid comparison of vehicles using both particulate trap aftertreatment and diesel exhaust fluid.

These procedures have proved to produce excellent results when the tests are conducted properly. Using any one of them can provide the tester with real-world crosswind conditions, if the conditions exist during the time of the test. Fleets often will use one of these test procedures to determine a fuel-savings percentage for a device at a specific speed.

To help equipment owners determine how these potential savings actually apply to a given fleet, TMC’s Cost Modeling for Aerodynamic Devices Task Force, led by Fritz Marinko, has developed a fuel savings calculator, the RP 1118, Fuel Savings Calculator for Aerodynamic Devices, which appears in TMC’s 2016-2017 Recommended Practices Manual.

RP 1118 provides equipment operators with a supplemental, interactive mathematical tool to evaluate the potential fuel and economic savings of an aerodynamic device that has been tested using one of TMC’s fuel economy testing procedures. (The tool also may be compatible with rolling road wind-tunnel testing conducted over a variety of drive cycles.) The scope of this RP is limited to Classes 6-8 tractors coupled to commercial trailers of all types. The speed range covered is 40 mph to 75 mph.

“The purpose of RP 1118 is to educate fleets on the importance of always testing with some crosswind, much like fleets operate on a daily basis, and it is also intended to provide fleet owners and decision-makers with an interactive mathematical tool to evaluate the potential return on investment of an aerodynamic device in a fleet’s specific drive cycle,” Marinko said.

Vehicle wind test tunnels were originally built with fixed floors. However, all modern tunnels now use a moving ground approach to better replicate conditions on the road. To capture the effect of this testing on fuel economy, it’s very important that — rather than just focus on the aerodynamic drag straight ahead — a range of yaw angles are tested that incorporate crosswinds and most accurately simulate real-world conditions, a premise upon which RP 1118 is based. EPA SmartWay has developed its own standardized wind tunnel procedure that correlates well to tests conducted using TMC’s fuel economy testing procedures.

The calculator requires the following information to offer a result:

• Average miles traveled per year per truck.

• Average fuel economy per truck.

• Cost of fuel per gallon.

• Number of trucks to be evaluated.

• Number of trailers to be evaluated.

• The percentage of miles traveled at speeds less than 40 mph and more than 70 mph.

• The percentage the device saved at 65 mph or other speed tested.

Given this information, the calculator will report:

• Projected fuel economy.

• Fuel savings in tenths of a gallon.

• Average fuel savings per truck with the device on the trailer.

• Annual savings per year per trailer using a given aerodynamic device.

The calculator was developed using Microsoft Excel. The Excel file may be downloaded by TMC members in the RP Library from TMC’s collaborative work platform, TMCConnect. (

The RP itself offers examples of how the calculator may be programmed for a given result. It also details the formulas that are used in the accompanying Excel spreadsheet. It takes about 10 minutes for the manager of a typical fleet to complete the calculator, Marinko said.

RP 1118 helps fleet managers determine how a given aerodynamic device might perform in terms of fuel economy in a given application, helping to take the guesswork out of spec’ing for fuel economy.

Marc Clark is manager, worldwide fleet equipment engineering, innovation/reliability, for FedEx Express. He is a former TMC general chairman and treasurer and received TMC’s highest honor, the Silver Spark Plug.