Does putting your car in neutral at red lights save gas

Putting your car in neutral at red lights sounds like it should save gas and improve mileage of your car. This intuitive notion comes from the concept of manual cars, where the clutch must be depressed or shifted to neutral while the car is idling.

If you are reading this you probably own a car with automatic transmission and don’t feel like keeping your foot on the brake for all those minutes at red lights. You may also be aware that automatic transmission cars generally consume more fuel than manual cars. So you probably want to get the most out of every drop of fuel from your tank, especially when gas, petrol, and diesel are becoming increasingly expensive and precious commodities.

Are automatic transmission cars less fuel efficient than manual transmission cars because of the whole idling in neutral concept? Here’s what we learned:

Keeping your automatic transmission car in neutral does NOT save fuel. In fact, it may even increase your overall fuel consumption. Here’s why.

Gas saving Experiment putting gear in neutral

We conducted an experiment to test this myth. We drove our car on a full tank for an entire month, and recorded the number of kilometres covered on each tank.

The car drove on more or less the exact same route everyday and encountered the same number of red lights.

When driving with the rule to shifting to neutral at every red light, the recorded kilometres were 400km for the fuel tank.

When driving without shifting to neutral at red lights, the recorded kilometres were 420km!

These are definitely non intuitive results, so we further investigated why this is happening.

It seems that car manufacturers are aware of the driving behaviour of most normal people, and have designed their cars to function according to this behavior. If most drivers keep the brake depressed at red lights, automobile manufacturers have made an adjustment for this situation, and made their engines reduce the rotations per minute, or rpm, when the car is idling with the transmission in Drive mode, or D.

Test Results

In the very car that we used for our experiment, notice in the images below how the rpm is lower when the gear is in D, compared to when the gear is in N.

It may only be a few points, but each point represents 200 rpm. Depending on the size of your engine, 200 rpm means that the volume of your engine’s combustion chamber has been filled 200 times for that point in a minute.

In our case, we were using a car with a 1.3 litre engine. This means each engine rotation amounts to 1.3 litres of volume. 1.3 litres x 200 rpm = 260 litres in a minute. While this volume is mostly air, small amounts of petrol gas or diesel are being sprayed into this space every minute.

So in our case, putting the car into neutral meant raising the engine’s rotations by 200rpm or 260 litres per minute. Either way more fuel is being consumed.

In conclusion, if you want to save gas expense on your car, better to keep it in D than shifting it to Neutral. Depending on your car’s efficiency, you may save a few litres every refill, or get more kilometres out of your tank. Over the course of a year, this little habit could save you up to $50 or even more per year.

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