Engine Heat: A math lesson that could save your life this winter

Easy numbers to calculate how long you can idle if you need your car for heat

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Let’s turn your car’s fuel gauge into a clock.


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Weird, right?

But consider this: do you know how long your car’s engine can run on a litre of gasoline? And do you know how much gasoline your car’s tank holds?

If you do, you’re a very simple math lesson away from being able to calculate how many hours of heat you can generate from your vehicle’s current fuel supply during survival idling — that is, running your engine to keep you warm if you’re helplessly stuck or stranded in the cold, and trying to stay alive until help arrives.

Whether you suffer a mechanical failure, accident, or other setback that sees you stranded in the cold, this little exercise can help you more calmly navigate things by removing an important unknown from the situation: how long can you stay warm for?


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Two caveats before our math lesson. First, it applies to modern fuel-injected gasoline engines. Second, nothing I’m about to explain takes higher importance than being properly prepared for winter driving emergencies, which includes keeping functional emergency survival gear and supplies in your vehicle at all times.

Here’s the math:

Take your engine’s displacement in litres, and multiply by the number 0.6. The resulting number is how many litres of gas your car needs to idle for an hour.

Let’s use the Lexus IS300 AWD as an example. It has a 3.5 litre engine. Multiplying 3.5 by 0.6 gives us the number 2.1. That’s roughly how many litres of gas you need to generate one hour of survival heat, by idling the engine. Put another way, every hour of running the engine for warmth reduces the fuel level by 2.1 litres.


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Now, let’s turn the fuel gauge into a clock. For that, you need the fuel tank size. That’s a quick web search away, just like your engine’s displacement, by the way.

The Lexus has a 66 litre fuel tank, and every hour of warmth uses 2.1 litres of fuel. If we divide 66 by 2.1, we arrive at the number 31.  That’s how many hours of heat you’ve got, if your fuel tank is full. From there, check your fuel gauge. Half full means about 15 hours of heat. A quarter tank means less than 8.

Toyota's Winter Event

In a GMC Yukon with the 6.2 litre engine, the math gives us 6.2 x 0.6 = 3.7 litres per hour of idling. With a roughly 91-litre fuel tank, it’s 91 / 3.7= 25 hours of heat on a full tank.

Ford Mustang with 5-litre engine and 61-litre gas tank? The math gives us 3 litres per hour of heat, and about 20 hours of heat on a full tank.


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So, if you drive something sporty — say, with a big engine and a smaller fuel tank, your clock may have relatively less survival idling time on it, compared to something with a smaller engine and a (relatively) larger tank.

For instance, the Honda CR-V. With 1.5 litre engine, it’s under 1L of fuel required per hour of heat. If its 53-litre fuel tank is full, it’s good for 59 hours of heat.

So, whether your car has a big tank and little engine, little tank and big engine, or something in between, you now know how to do the math for yourself, thereby eliminating one stressful unknown if you ever get stranded in the cold. Consider this your annual reminder to keep those fuel tanks (and batteries) full for your winter travels. 


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Nina Zatulini

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