Making your own short-term test

The following suggestions for conducting fuel economy tests are highlights, summarized from SAE test method J1376. More details are available from the Society of Automotive Engineers, Bridgestone or your tire supplier.

The Maintenance Council and SAE also have other fuel economy test procedures that you may find useful. Consult TMC or SAE for more details.

Tractors

Use two tractors from your own fleet which are as close to identical as possible. The make, model, engine, drivetrain and optional equipment should be the same; both tractors should be fully broken in and have about the same mileage, and all operating conditions should be identical.

Before testing, each tractor should have a thorough check and tune-up, including replacement of all fluids and filters and alignment of both steer and drive axles.

In Bridgestone tests, fan clutches are locked “on,” air conditioners and radios are turned off, headlights and running lights are on and windows are kept rolled up during each run.

Short-term tests can be done quickly, and can be highly repeatable. They require careful equipment selection and preparation, a high level of personnel involvement and training, and may even require modifications to test vehicles.

Fuel system

Bridgestone recommends that you modify the fuel system of each tractor so that you can supply fuel using a detachable auxiliary fuel tank. At least two detachable tanks, of about 12–18 gallon capacity with quick disconnects for the fuel supply and recovery lines, will allow accurate measurement of fuel consumption during each run.

If you use small tanks in your tests, an external fuel cooler on the recovery line is recommended to help reduce the temperature of the fuel in the test tanks.

Use quick disconnects to substitute your test tank for the main fuel tanks, and if your test tank is small, an external fuel cooler is recommended.

Trailers

Choose two trailers that are as identical as possible, and which are loaded to the same weight normally used in your operation. Age and condition of the trailers should be matched, and a check and alignment of the trailer axles is recommended.

The load should be distributed to place equal weight on each of the drive and trailer axles.

Tires

For your “baseline” run, you will need the same tires on both trucks. Obviously, results will be more meaningful if you use the same brand, model and sizes you normally use in your operation. Of course, you should use your recommended inflation pressures at all times.

Use new tires, but run them for about 100 miles at your test speed to break them in before testing. Likewise, before each set of tests is run, a one-hour “warm-up” of the vehicle and tires is recommended.

Once your baseline run is completed, you will change tires on the test vehicle only. The control vehicle should continue to use the same tires as during the baseline run.

Fuel supply

Use whatever brand and type of diesel fuel you normally do, but secure a large enough supply so that you can match fuel quality during all your tests.

Drivers

Assign the same driver to each vehicle throughout testing. Drivers should be thoroughly briefed on test procedures, especially on the importance of consistency in their driving practices.

Pretest checks

Before beginning a test series, use accurate scales to verify both the gross combined vehicle weight as well as individual axle weights. Match the vehicles as closely as possible.

Weigh the units with the drivers and allow no passengers, unless they will ride with the drivers throughout all tests. A variation of about ±50 lb. between the same axles on two different vehicles is considered an excellent match.

Test route

While Bridgestone conducted tests on a closed track, you may find it more convenient to use nearby highways.

Try to select a route that is as close as possible to 15 miles (so that a complete loop will be 30 miles), and which is reasonably level, with a roadbed in good condition and minimal competing traffic. Avoid areas with high winds or sharp curves.

Short-term tests involve careful preparation, rigidly controlled procedures, special personnel and excellent communications systems. Short-term fuel economy testing controls and eliminates variables for relatively high precision and repeatability.

Test runs

Each test consists of three runs by both vehicles over the 30 miles of the course.

Both trucks should make their runs at the same time, but delay the start of the second vehicle by at least 45 seconds to eliminate the effects of “drafting.” This way, both vehicles will be affected equally by wind, road and traffic conditions.

Each driver should have a stopwatch to time the total length of each run, to assure the interval between the vehicles and to time a 3-minute idle period.

In closed track tests, Bridgestone drivers idle for one minute at the start of the test, one minute at the 15-mile mark, and one minute before shutdown, but you may incorporate this idle time as is practical for you, so long as a 3-minute idle period is included in each run.

After each stop, drivers should accelerate as quickly as possible, without over-revving, to whatever speed you normally use on the road, and hold that speed as closely as possible until the next stop or idling point. Braking for stop and idle points should be consistent with safety and normal operation.

At the end of each run, test personnel should wait at least 10 seconds after engine shutoff—for fuel drainback—before removing the test tanks.

If additional pre-weighed test tanks are available, these can be immediately coupled to the fuel system to allow a new run to begin, saving time in the test process.

A two-way radio system will facilitate communication between test personnel and drivers.

Test personnel should also use stopwatches to verify total run time, delay between first and second vehicle, idle times and fuel drainback intervals.

Monitor the ambient temperature, wind direction and wind speed during each run. High winds, gusty conditions, or radical temperature changes may adversely affect repeatability of tests.

Fuel consumption measurements

Because fuel volume varies significantly with temperature, Bridgestone recommends that you weigh test tanks before and after each run to determine the quantity of fuel used.

A scale accurate to 1/20th of a pound is ideal, but the more accurate the scale, the more accurate the results.

Reporting and calculations

At the end of each run tabulate the data, using a separate form for each vehicle.

Calculate the ratio of the test vehicle’s fuel consumption to the control vehicle’s fuel consumption for each run and record it on the form.

This ratio, called the “T/C ratio,” should then be averaged, and the average recorded on the form.

If the range of T/C ratios for the three runs does not exceed three percent on average, the test can be considered valid.

If the range is greater than three percent, variables are not sufficiently under control, and additional runs must be made until results are repeatable—or the problem is found.

Once the results from the baseline runs are acceptable, you can begin making changes to the test vehicle. Your final T/C average from the baseline runs will be used to evaluate the results of subsequent tests.

Bear in mind that if you change too many things at once on the test vehicle, you may not be able to determine which change produced the effects you observe.

Conduct your test runs in exactly the same way as your baseline runs, recording data and making the same calculations and repeatability checks.

By running the control vehicle as before, you can compare your control vehicle results during these runs with your baseline results, to assure that consistency is maintained.

The results of short-term fuel efficiency tests are almost impossible to duplicate in real world, day-to-day operations. Typically, your normal operations will produce no more than half the fuel economy measured in controlled testing.

Test run calculations

Your test run T/C ratios can now be compared with those from the baseline runs to determine the effect on overall fuel economy of the changes you have made.

By dividing the difference in T/C ratios by your test run T/C ratios and converting the result to a percentage, you can see what percentage change your changes produced.

This method of calculation automatically makes your baseline run become 100 percent. As an example, if your percentage comes out to 8 percent, this means that your test run gave you 8 percent improvement in mpg over your baseline run. When you use this method, larger numbers always mean better fuel economy.

Will I get the whole amount?

In a word, no. Tests like these, conducted under near-ideal conditions, always produce larger changes than can be achieved in normal day-to-day operations.

In the real world, drivers encounter hills, wind, bad weather, heavy traffic, and a host of other conditions that will reduce fuel economy.

Nevertheless, this “Snapshot” method will give you a fast look and help you identify trends, because it controls variables and is relatively quick to perform.

The results of this snapshot test should give you an idea of whether or not a change to more fuel-efficient tires will show up on your bottom line.

Remember, Bridgestone experience indicates that if you don’t see at least a two percent change in fuel economy during testing, you probably will not be able to see any change in fuel economy in normal operations.

And, whatever change you measure in the test, you probably shouldn’t expect to see more than about half that change over the long term.

Regardless, whether you try the snapshot test or not, Bridgestone recommends that you begin tracking your fuel economy under long-term, real-world conditions.