AAA: Don’t Trust “Miles To Empty” Gauge

If you’re one of those people who finds themselves watching your digital fuel gauge count down your “miles to empty” before coasting into the gas station on fumes, you’ll want to heed a new warning from AAA. The American Automobile Association says those gauges aren’t always accurate, so it’s best not rely too heavily on them. Your less risky option, researchers say, is to fill up when you hit the one-quarter of a tank mark.

AAA researchers studied in-dash fuel economy displays that give you “miles to empty” or estimate the number of miles a vehicle gets per gallon. Testing on these systems found that their estimates vary significantly over shorter trips and depend on the consistency of various factors that impact gas mileage like speed and acceleration. Researchers say relying too much on them could put you at risk of running out of gas. And many people do rely on them. According to an AAA consumer survey, 74% of drivers use their “miles-to-empty” display when they are low on gas to decide when to fill up.

“People want to get the most out of a tank of gas, especially when prices are higher,” said Megan McKernan, manager of the Automotive Research Center. “Collectively, the systems we tested were relatively accurate, but a closer examination of different driving scenarios revealed significant variability based on changes in speed, acceleration and distance.”

Driving Habits Directly Impact Fuel Economy

Driving habits can impact fuel economy and therefore the accuracy of the digital fuel gauge in your vehicle. To put that to the test, 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.

Here’s what researchers found:

• 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. AAA says the 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, AAA says 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.

“We ran our test vehicles through different driving situations ranging from cruising at highway speeds to being stuck in traffic to typical city driving,” said McKernan. “Despite the irregularities our testing found, a vehicle’s fuel economy display is an important tool to understand how different driving styles impact how efficiently a vehicle uses fuel.”

Maximizing Fuel Economy is Key as Gas Prices Reach 7-year High

AAA is also taking this opportunity to address the recent rising gas prices they’ve now reached their highest point in seven years. To offset some of those costs, AAA recommends 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.

AAA also offers another last reminder that it’s best to keep at least a quarter of a tank of gas in your vehicle in case of of unexpected delays. Doing this will also help prevent fuel pump damage that can happen when a vehicle’s gas tank is regularly run down to empty.