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Are miles-to-empty indicators accurate?

Photo by Jen Warren Photo by Jen Warren

I once owned an ancient Volkswagen Beetle that had no fuel gauge. If the engine began to sputter for lack of gas, VW instructed the driver to quickly flip a lever under the dash that provided an additional bit of fuel, presumably enough for the car to reach a gas station.

I once forgot to reset my Beetle’s lever after a fill-up, and when the engine sputtered, there was no reserve. I was pushing 65 mph on the freeway with a big semi nipping at my exhaust pipe as my engine was dying, causing me to anticipate my own demise unless I pulled off the road fast enough.

Today, it’s unimaginable that a car wouldn’t have a basic fuel gauge. Most modern vehicles also have electronic doohickeys that calculate a car’s fuel economy and show how many miles remain before the tank runs dry. Despite all this information, folks still run out of gas—AAA’s Roadside Assistance gets about a half million such calls a year.

Running out of gas can be dangerous for you and your passengers—and for your bank account, too. Unlike my primitive Beetle, which had a mechanical fuel pump that wasn’t damaged by a lack of gas, modern cars typically have an electric fuel pump buried inside the gas tank that is lubricated and cooled by the gasoline. The electric pump serves a finicky, finely tuned fuel-injection system. Without gasoline, the electric pump can self-destruct, sending debris into the fuel injectors and requiring costly repairs.

Read more: Is there a difference in the quality of gasolines?

What an AAA survey found

Because a AAA consumer survey indicated that 74% of drivers use their miles-to-empty displays to decide when to fill up, the accuracy of the readings is important. AAA, in collaboration with the Automobile Club of Southern California’s Automotive Research Center (ARC), tested the displays on 16 vehicles to learn more.

 “We ran the vehicles through different driving situations ranging from cruising at highway speeds to being stuck in traffic to typical city driving,” says Megan McKernan, the ARC’s manager.

The ARC found that miles-to-empty readings varied significantly for the cars as they progressed through different driving cycles. That’s no surprise, because fuel economy differs depending on the cycle—as, for instance, in city versus highway driving. When driving conditions change, the displays likely lose accuracy until their internal algorithms adjust to the new driving conditions.

The displays are likely to be most accurate under consistent driving conditions. Moreover, the ARC found that all the vehicles underestimated miles-to-empty, with between 6 and 55 miles of travel remaining when the displays showed zero.

Which car is most accurate?

Regarding fuel-economy displays, only a Mercedes-Benz CLA was spot-on accurate. A Mini Cooper’s display overestimated its fuel economy by 2.2 mpg. A Kia Optima’s display was not optimistic, underestimating the car’s fuel economy by nearly 1 mpg.

None of this suggests that fuel-economy and miles-to-empty displays aren’t useful tools for a driver. But there’s enough variability that you shouldn’t bet your life—or that of your car’s fuel pump—on them.

AAA’s advice is simple: Rather than risk running out of gas, refuel before the gauge reaches the one-quarter mark. And if gas stations run dry because of supply issues, or if you must evacuate because of wildfire, earthquake, or major storm, you’ll be glad you had fuel in the tank.

Veteran automotive journalist Peter Bohr has been writing about cars for more than four decades.

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