Be wary of vehicles' 'miles-to-empty' estimates

Jul. 28—ATLANTA — Drivers could be taking an unnecessary risk if they over-rely on fuel economy displays.

AAA tested the accuracy of in-vehicle fuel systems that measure fuel economy and range (miles-to-empty). Collectively, the systems tested were relatively accurate, but a closer examination of different driving scenarios revealed significant variability based on changes in speed, acceleration and distance.

"People want to get the most out of a tank of gas, especially when prices are higher," Montrae Waiters, a spokeswoman for AAA—The Auto Club Group, said. "Unfortunately, these systems aren't perfect, so if drivers become too reliant on them their vehicle may run out of gas sooner than anticipated."

AAA, in collaboration with the Automotive Research Center (ARC) of the Automobile Club of Southern California, used a dynamometer, essentially a treadmill for vehicle testing, to run selected vehicles through a series of simulated driving scenarios to determine the accuracy of the fuel economy estimation and range value (aka "miles-to-empty") systems.

On average, the fuel economy display of the vehicles tested showed a relatively low error of 2.3% as compared to the fuel economy measured by the dynamometer. However, individual vehicle error varied greatly, ranging from -6.4% to 2.8%. The negative number indicates that one test vehicle overestimated fuel economy by 6.4% or 2.2 mpg, while another underestimated it by 2.8% or 0.9 mpg. These specific results suggest that each vehicle reacted to changes in driving differently, and that the accuracy can be impacted by driving style and conditions.

Testing of the "miles-to-empty" display found similar results with accuracy fluctuating across driving scenarios. While each manufacturer likely uses a unique algorithm to estimate vehicle range, it can be assumed that some historical driving data is also used to determine the vehicle's fuel efficiency for future driving. Therefore, the range estimation, at any given point, is affected by the vehicle's most recent driving conditions.

The information displayed by these systems can give drivers a clearer picture of how their specific driving habits influence their fuel economy. To do this, drivers should reset their vehicle's trip data after filling up, and then watch how their fuel economy display changes as driving conditions change.

In recent weeks, gas prices have reached their highest point in seven years. To offset some of this additional cost, AAA recommends drivers do the following:

— Plan ahead and run multiple errands in one trip, and whenever possible avoid times of day when traffic is heavier.

— If you own more than one car, use the most fuel-efficient model whenever possible.

— Avoid hard acceleration to maximize fuel economy, and always inflate your tires to the recommended pressure found inside the driver's side door or owner's manual.

— Remove unnecessary and bulky items from your car. Minimize your use of roof racks and remove special carriers when not in use. Smaller cars weighed down by heavy cargo will have a greater reduction in fuel economy than larger models designed to carry more weight.

— Consider minimizing your use of air conditioning. Even at highway speeds, open windows have less effect on fuel economy than the engine power required to operate the air conditioning compressor.

— In hot weather, park in the shade or use a windshield sunscreen to lessen heat buildup inside the car. This reduces the need for air conditioning (and thus fuel) to cool down the car.

While these tips will help improve fuel economy, AAA reminds drivers it's best to always have at least a quarter of a tank of gas. This will ensure drivers have enough fuel in case of unexpected delays but also helps to prevent fuel pump damage that can occur when a vehicle's gas tank is regularly run down to empty.