While some cars tell the truth about their lack of fuel... While some cars tell the truth about their lack of fuel, others will give you a few extra gallons. (Creative Commons photo by t3mujin)
Warning lights in cars work almost like the electrodes scientists use to test responses in lab rats: a current is run through them, and the subject reacts in some novel way to the stimulus. Leading the way for dashboard stimulation – as far as cautionary alerts go – would be the Check Engine light and the Low Fuel light.

The check engine light might inspire the most profound responses because it could mean just about anything, from a faulty sensor to imminent vehicular implosion and a gargantuan repair bill. It's like the doctor telling you he has to run some tests before he knows what's wrong with you, but it could be either an ear infection or a brain tumor.

It is perhaps the low fuel light, however, that is the most legendary and creates the widest range of responses and means the most to the most people. There is practically an industry built around the tiny stencil: it has its own website, it brings up more results on Yahoo and Google than other warning lights, there have been mantras created to defer its consequences, it has been called a liar and worse, its accuracy is tested more often and more vigorously than a law student's and Jackson Browne sang a song about it (Running on Empty). It has probably been responsible for more appeals to a higher power than any car-related activity save love by the dashboard light. That's quite a following for one tiny bulb.

The cult of the Low Fuel warning light is strengthened by the fact that few people have any idea what it really means when the light is illuminated. Your car's owners' manual might tell you how many miles you can go when you hear that warning chime, but that's just a handy thumbnail – how far you can actually go depends on how much gas remains in the tank and where and how you're driving.

In fact, even the people who make the cars aren't always sure how much gas is left in the car when the red flag goes up. That's why some other owner's manuals, such as those for Honda and Acura vehicles, won't even guesstimate the distance to truly empty.

"The amount of gas left in the tank when the low-fuel warning light illuminates," said Honda's Chris Martin, "varies greatly from car to car even within a particular model line." So just because you are intimately acquainted with the camel-like habits of your Honda Civic after having tested it a few times doesn't mean you can expect the same or better performance from the smaller Honda Fit.

"It's impossible to say exactly how much is in the tank when the light comes on," continued Martin. "For that reason, we recommend that a driver attempt to refuel as soon as possible when the low-fuel indicator illuminates."

Combine "impossible to say exactly how much" with normal variations in driving that include tire pressure and weather. Then, add the fact carmakers still want the car to start, even if it's low on fuel, when it's at an incline – like driving and parking in San Francisco. Finally, note the fuel gauge isn't the most precise instrument our civilization has created; we've driven several cars that ran flawlessly at 70 mph for miles when the needle was below the 'E'.

Forums are full of drivers commenting on how much gas they have in their car even after the warnings have seemingly reached Def-Con 1. It is no wonder, then, that when folks talk about how far they've driven with the low fuel light on they sometimes speak as if it were an Act of God. A Canadian journalist who ran several cars until they stopped got 98 miles in a Hyundai Elantra after the warning light, but just 58 miles in a Kia Spectra -- which is a smaller car utilizing the same engine as the Hyundai. Another driver actually got angry when he was able to get his Dodge Grand Caravan another 110 miles on a supposedly nearly empty tank.

And as hybrids gain market share, that kind of distance-after-'E' won't be so strange. Running out of gas in a hybrid has the potential to severely damage the hybrid system, so carmakers will be especially cautious about making sure consumers have time to get to the pump. Hard numbers about the remains of the tank still aren't easy to come by, but Lexus tries. "One to three gallons," said Lexus' Bill Kwong, "depending on the model."

That, however, is about as far as they'll go when it comes to pinning things down. On the matter of how far you could actually go with your one-to-three gallons, it was back to general terms. "If the vehicle provides a 'distance to empty'," said Kwong, "it's a conservative estimate of what's left in the tank based on a conservative estimate of the current tank mpg. The distance to empty for the driver will vary based on the vehicle, load, road condition, weather, tire pressure and driving style."

Mercedes offers a much more precise look at how much gas you have left: owner's manuals provide a chart of how big your car's gas tank is and how many gallons -- approximately -- you have left when things get dicey. The GL, for instance, has a 26.4 gallon tank and around 3.4 gallons in reserve. The C-Class has at 17.1 gallon tank and keeps about 2.1 miles in reserve. Taking the average mileage for both vehicles, you should get 60 miles in the base model GL Bluetec, and 45 miles in the C300 Luxury Sedan.

That is, until you read the caveats. "The mileage number is monitored by the engine management system and takes into account current fuel usage and driving style to calculate remaining mileage," said Mercedes' Nicole Weiss. "This number is also very sensitive in the reserve range and changes drastically based on throttle demand."

Weiss went on to enumerate the steps you should take if you want to prolong what little gas you have left. "Turn off the air conditioning," she said. "Close all windows if possible, reduce your speed, no jack rabbit starts from standstill and only enough [power] to keep the vehicle at speed. Correct tire pressure positively affects the overall fuel consumption."

Nevertheless, the safest bet is the one offered by all three carmakers, nearly verbatim: in the words of Bill Kwong at Lexus, "When the low warning light turns on, the driver should really fuel up as soon as possible."

Of course, a heartfelt prayer and that mantra probably won't hurt, either:
Gods of Petrol, fuel, and gas
Expand my fuel and do it fast!

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