Which Type of Fleet Fuel Economy Test is Right for You?

More and more fleets are focusing on improving their fuel economy as a part of their sustainability efforts. However, in order to make proper decisions about which technologies to invest in, fleets need reliable fuel economy test results. 

There is a variety of test methods available for fleets to choose from, and each has its pros and cons. Understanding each test method will allow fleets to choose the test method that is best for them.

SAE J1321 Type II Testing

One commonly used fuel economy test is the SAE (Society of Automotive Engineers) J1321 Type II Testing protocol. This comparison study uses real vehicles and bases the results on weighing fuel tanks before and after the test and providing a percentage change. Two fleet vehicles take part in a dedicated test that can be conducted on a track or highway and can be used to measure aerodynamic devices, tires, lubricants, fuel treatment/devices, gear ratios, air intake technologies, engines and transmissions.

Fleets can conduct these tests themselves after purchasing testing equipment and training employees in proper test procedures. They also need to find a suitable location in which to conduct the test. The test can also be outsourced, for a price.

Accuracy (margin of error) has been a concern with this method in the past. The common accepted margin of error in the trucking industry is a ± 1.0%, but this method can reach ± 5.0% error and still be within the test parameters. This makes it very important to calculate the margin of error when considering the validity of the results.  

SAE J1321 Type II Testing Pros:

• Fleet can test themselves with purchased test equipment, training and a suitable location.
• Fleets get answers in one or two days on real fleet vehicles.
• The tests can be conducted for low cost if conducted internally, but relatively high costs can accrue if outsourced.
• There are no complications as can be found with in-service testing including issues with labor, other priorities and vehicle issues.

SAE J1321 Type II Testing Cons:

• Questionable accuracy can be a problem if the test is internally conducted by an untrained fleet. 
• As noted, the results are not verifiable.
• The test is not able to provide actual fleet savings values.

Scale Models and Wind Tunnels

Wind tunnel tests can provide quick and repeatable results, but they test only aerodynamic technologies. Wind tunnels can simulate highway driving conditions using scale models that are either one-eighth, or one-third of the actual vehicle size in North America. (The size of the wind tunnel determines the size of the model.) Another limiting factor is that wind tunnels that can fit a full-size tractor are not long enough for a 53-ft trailer.

It's important to note that wind tunnels measure aerodynamic drag, not fuel savings. The common practice for translating wind tunnel results to fuel savings is to multiply the final drag coefficient achieved by the test model by 50%. However, this only gives you a rough estimate and does not account for variables such as weight, terrain, driver behavior or duty cycle.

Wind Tunnel Pros:

• Fast and easy to make changes to vehicle configurations.
• Real vehicles or fleet vehicles are not necessarily required.
• Cost-effective for hired testing.

Wind Tunnel Cons:

• Fleet vehicles are not necessarily involved.
• Can only be used for aerodynamics.
• The wind tunnel vehicle model may not match fleet vehicles.
• The test does not provide actual fleet fuel savings.
• The test may require building custom scale model parts.
• It does not measure fuel saved.

Missouri-based motor carrier Prime is constantly evaluating new products to continue to improve its fuel economy, says Chris Holtmeyer, manager, fleet maintenance for Prime.

“You have to do your due diligence when it comes to new products,” he says.

Holtmeyer explains that fleets need to be aware of the margin of error for various kinds of testing. He uses the example of wheel covers, where fuel economy gains can be 1% or less.

“A lot of fuel economy tests have a 1% to 2%± margin of error,” he explains. “So, you have to have a lot of confidence in the test procedure, otherwise you will not see the fuel savings you expect.”

Using wind tunnel testing, you are able to configure your equipment the way you want relatively quickly, as compared to doing it in track testing, Holtmeyer adds.

“In real-life testing changing skirts out, installing deer guards, changing our rear tail devices and mud flaps takes a lot more time. When we did wind tunnel testing, they were able make many changes quickly and were able to do things like adjust the positioning and angle of the trailer skirts.”

Another consideration whether to do a wind tunnel test vs track testing is the consistency of the tests wind tunnels can produce, Holtmeyer adds.

“In real life, variables like weather, wind, etc., can alter outcomes. So multiple runs may be required to achieve desired results,” he says.

Computational Fluid Dynamics Tests

Computational Fluid Dynamics (CFD) tests are computer simulations that measure simulated aerodynamic drag. CFD tests rely on highly detailed geometry of the truck and components being tested as well as enormous computing power that is often provided by offsite servers.

These tests are not limited in physical size and therefore do not require the compromise of scale models as in wind tunnel tests.

CFD software is easily accessible, but results are highly dependent on the inputs of the operator. This makes finding a reliable provider extremely important. Like wind tunnel results, CFD measures aerodynamic drag, and a 50% multiplication factor is applied to estimate fuel savings.

CFD Pros:

• Fast and easy to get answers and make changes.
• Real vehicles are not required to complete testing.
• Cost-effective for hired testing.

CFD Cons:

• The test is only for aerodynamics.
• The model vehicle used in the test may not match fleet vehicle or creating a match for the fleet can incur additional cost.
• It does not provide actual fleet savings.
• The test requires building custom digital model parts for testing.
• The test does not measure fuel saved.
• Test results are reliant on operator inputs.

“CFD allows you to look at how air flow passes from the front of the vehicle, through the vehicle, and off the back of the vehicle,” says Naethan Eagles, technical director of Total Sim LLC, a full-service provider of CFD services. “You can also see how the drag is building up, the interactions that are happening across the vehicle, how specific components or add-ons work, and how the truck interacts with the trailer.”

In addition to your drag numbers, CFD also gives you other information that helps you understand what is going on so you can make informed decisions about your aerodynamic set up, Eagles adds.

“Do you want skirts, for example?” he says. “Boat tails? Wheel covers? Flaps? Gap devices? CFD gives you insight into how things work together.”

Eagles also notes that there are rules and best practices that govern CFD and says that if these tests are conducted properly, very good and reliable test results are possible.

Coast-Down Tests

Coast-down tests use real vehicles and measure the changes in speed while coasting with and without a technology installed. The values obtained from these comparisons are then entered into a math formula to predict fuel savings.

This test can be used mainly to test aerodynamics and tire rolling resistance. And, since the vehicle is coasting during testing, it cannot be used to evaluate engine or drivetrain related technologies. Special test equipment is required for coast-down testing to measure aerodynamic influence, weather conditions, and vehicle speed.

Coast-Down Test Pros:

• Fleets can test themselves with purchased test equipment, training, and a suitable location.
• Fleets get answers in one or two days that are based on real fleet vehicles.
• If conducted internally, there is a low cost for the test.

Coast-Down Test Cons:

• The tests can only be used to measure aerodynamics and tires.
• It does not measure fuel saved or provide actual fleet fuel savings.
• Test equipment must be purchased and learned to conduct this test.
• Costs are relatively high if the tests are outsourced.

In-Service Fleet Tests

In-service fleet tests are the go-to evaluation methods for many carriers. Fleets can monitor similar trucks on similar routes but with the addition of a fuel saving component on one group of trucks or trailers. This allows the fleet to compare the performance of their trucks with and without the device, and almost any technology can be tested.

Typically, data from in-service fleet testing is collected over a long period of time — anywhere from 6 to 12 months. Another important point regarding these tests is that unless the trucks are spec’d exactly the same before adding the device to be tested, it can be difficult to get accurate results.

Many fleets have used this method for gauging fuel economy. But very few are confident of the decisions they reach because of complications such as operations, labor, priorities, variables and margin of error.

A good source for details on running in-service fleet tests is the American Trucking Associations' Technology & Maintenance Council’s RP 1106(T), which outlines recommended practices for conducting in-service fleet testing.

In-Service Fleet Test Pros:

• Test results always match fleet's own equipment/operations.
• Measures actual fleet fuel savings.
• Fleet can test technology themselves.
• Provides durability, maintenance feedback simultaneously.
• Low overhead cost to conduct test (if not including labor or time in total).

In-Service Fleet Test Cons:

• It takes anywhere from six to 12 months to get needed answers.
• A minimum of 10 vehicles in each of the “test” and “compare” groups (known statistically as “sample size”) is required.
• There is a large margin of error with sample sizes less than 100 vehicles.
• A large product investment is required to conduct the test.
• The test requires a great deal of labor on the part of fleet personnel.
• The test is subject to failure due to fleet higher priorities and complications of in-service operations testing.

“We call it real-life testing," says Tony Morthland, director of maintenance at Illinois-based carrier Nussbuam. “We did an SAE test in the past, but it did not give us real life results because there are so many variations in the real world such as wind, weather and other factors.”

Morthland says that Nussbaum does run in-service fleet tests over a long period of time to account for all those variations, though.

“We did a test where we compared dual tires to wide-base tires,” he says. “We ran one tire type for a month, collected data and then switched the tires and ran the trucks for another month to get a baseline.”

During the tests, Morthland says drivers document the fuel they use and Nussbaum’s IT department analyzes all the data and issues a report on the results.

“What we have found is that the reports based on data from the tests are very accurate in predicting real life savings,” he adds.

Holtmeyer says while Prime does not typically do its own in-service testing, it does partner with suppliers to do track testing.

“We like to take a scientific approach,” he says. “We put it in the hands of the professionals creating which allows us to get good data compared to a baseline.”

MVT Solutions Tests

MVT Solutions tests use real vehicles, measure real fuel saved and can test nearly any technology type. Testing is done either on a track or highway, depending on the technology type using customized sensors and data recorders that account for variables like temperature, wind and vehicle weight. Results can be translated to a fleet’s in-service operations using fleet telematics data. Accuracy is typically ± 0.2 % to ± 1.0% and can be verified with digital data, which cannot be done with weigh tanks.

MVT Solutions Test Pros:

• Test results are available the same day.
• High accuracy and a low margin of error.
• Provides actual fleet savings values.
• Quickly filters out technologies that don't work for a fleet.
• Low cost compared to other hired testing on real vehicles.
• No complications like those seen in-service testing such as labor, other priorities and vehicle issues.

MV Solutions Test Cons:

• Fleet cannot perform tests without the involvement of MVT Solutions equipment and personnel.
• Some vehicle downtime occurs unless the fleet uses MVTS’ own test vehicles.
• Scheduling is subject to availability and weather like other for hire and real vehicle tests.

Reach Out to Experts

Before investing in a fuel saving technology, it is best to try to verify manufacturer claims. As this article shows, fleets have a variety of test protocols to choose from to do so. None are perfect, however. Each one has pros and cons, and it’s a matter of choosing which test can help your company best.

When choosing a test method, it’s a good idea to check the company’s track record first for fleets that endorse them because your peers are a reliable resource.

If you’re not sure which test is the best for your fleet, one option is to try multiple test methods to gather a more comprehensive picture of fleet fuel economy instead of trying to pick a favorite and relying on just one test.

When you start testing, don’t expect to learn everything — or anything. And don’t hesitate to contact experts for input on what you’re doing. However, it is also worth noting that the worst thing a fleet can decide is to do nothing because it guarantees wasted fuel, profits and emissions.

Daryl Bear is lead engineer and COO for MVT Solutions, a New Mexico-based subsidiary of Mesilla Valley Transportation that was founded in 2016, which provides fuel economy testing based on race car engineering. 

This article was authored and edited according to Heavy Duty Trucking’s editorial standards and style to provide useful information to our readers. Opinions expressed may not reflect those of HDT.