Can you trust the low fuel warning?

Although the first car (a Benz, if you’re curious) hit the road in 1879, the fuel gauge wasn’t invented until about a hundred years ago, in 1917. (It’s likely a lot of motorists ran out of gas during those intervening years!)

But fuel gauges weren’t enough, it seems. Because some years back cars also gained a low fuel warning light.

And it wasn’t long after that that the first driver decided to push it “just a bit more” when the Low Fuel light came on. (And managed to strand himself at the side of the highway.)

These days, advances in “gas station avoidance technology” continue to march along. Recently, manufacturers like GM have been adding range indicators to their cars—showing how far you can drive on the gas left in the tank.

But can you really trust these indicators? Here’s what you should know.

How the low fuel gauge actually works

Details vary, but most cars have a float in the gas tank. This is connected to a lever and “sender’ that puts out an electrical signal. That drives the gauge, and when it reaches a certain level it also turns on the low fuel light. If you park on a hill you may find the light comes on when first starting the car, only to go out once you get onto level ground.

Where to find those essential warning lights

Every Chev, Buick, Cadillac and GMC vehicle puts the fuel gauge directly in front of the driver.

In addition, once you get down to around 10 litres—although that number is higher in a vehicle with a big engine—a low fuel light comes on. It’s the yellow one, usually with a picture of a gas pump. Certainly you’ve seen it.

But on top of that, most modern GM vehicles have a multifunction display in front of the driver. You can use it to page through a series of screens to view information about what’s going on with your car. These screens include trip recorders, instantaneous and average speed—and, yes, fuel range. If you’ve just filled up with gas, you’ll see something like “Range: 570 km.”

How range is calculated

Modern engines precisely measure the quantity of fuel being injected. That’s to maximize efficiency and minimize emissions, but it also means your car knows exactly how much juice it’s guzzling at any moment—or sipping, as the case may be with cars like the Chevy Spark or Malibu Hybrid.

Your engine also knows how fast you’re driving, and can use that info to calculate your instantaneous fuel consumption. You can see it doesn’t take a lot of extra calculations to arrive at averages for speed and consumption either.

Range is calculated by computing your average fuel consumption and then comparing it with the amount of gas left in the tank. If there are 10 litres in there and you’re averaging 10L/100km, then it stands to reason that you’re good for 100km before the engine stops.

Or are you?

Can you actually trust that range number?

Honestly, it depends. If you’ll drive the next 100km exactly the same way you drove the last 50 you should be good. Lower your consumption by driving more gently and you could go a few extra km.

It gets tricky, however, when you leave the highway and start driving in the city. Then your consumption per km goes up—and those averages your car is using no longer apply.

Just to emphasize the problem with city driving and average fuel consumption, next time you’re stopped at a red light, click through to the instantaneous consumption page. You’ll see it displays something like, “- – -”. That’s because even though you’re still using gas, your speed is zero—and you can’t divide by zero.

In other words, those averages only apply when you’re on the move.

(Of course, drive a 2018 Equinox with the engine stop-start function and you won’t waste gas idling at traffic lights.)

What to do?

Let’s be clear: like the low fuel light and the gas gauge, the range number is just an indicator. None are precise, scientific measurements.

Think of those last few liters as a safety margin, but one you shouldn’t plan on using. When your car says you need gas, buy gas.

And if you don’t know where the nearest gas station is? Well, that’s when it pays to have GM’s OnStar service. Press that little button on the mirror and you’ll be connected with an assistant who’ll happily provide you with directions to the nearest gas station.

In any case, if you have questions about fuel consumption, gas gauges, or even warning lights, we’d love to answer them for you at Budds’.

Come see us! Just make sure to fill up your tank first.