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Coasting in Neutral with Automatic Transmission

I notice my instantaneous gas millage goes up if I coast in neutral in my new Volvo V50; say when cruising up to stop lights or coming down a hill. I appear to be saving gas, but am I damaging my automatic transmission?

Your instantaneous gas mileage gauge isn’t all that accurate in the first place. In the second, you’re shifting in and out of neutral, causing wear. In the 3rd, your entire car will be damaged if you can’t accelerate out of the way of the speeding truck because you’re in neutral. And finally, it’s illegal.

This has been discussed to death the hundreds of times it’s been asked in the past. Please search for those discussions, which apply to you as well.

Shadow: While I would agree with your 1st, 2nd (maybe) and 3rd points…illegal? So you’re telling me I can’t take my manual out of gear when approaching a light? That seems … well … wrong. I’ll admit I don’t know every rule of the road, but I know most, and this one is new to me.

It may seem strange, but many states have laws on the books forbidding coasting in neutral. These laws are antiquated, mainly applying to commercial drivers, but they are laws. It’s all about maintaining control of the vehicle.

When your car is coasting in gear (manual or automatic) your injectors are shut down as the momentum of the vehicle is turning the engine (no fuel needed). When you shift to neutral, the engine needs fuel to keep running. It’s also illegal and unsafe.

While it’s true that cars made in recent years do save fuel by shutting off the injectors when momentum of the vehicle is turning the engine, some “freewheeling enthusiasts” have argued they can save more fuel by putting it in neutral. This was in the context of coasting for long distances and not coasting to a light/stop sign.

Their argument was that even though the injectors shut off if they kept it in gear, the engine braking would cause the vehicle to not coast nearly as far as it would in neutral. They claimed they could get better gas mileage coasting long distances in neutral without the engine braking.

Their argument does have some merit.

While I don’t consider it to be a laboratory-grade measuring device, I have noticed that when I coast in gear, my vehicle’s instantaneous MPG readout goes WAY up.

If I coast in gear at expressway speeds, the readout actually goes up to 99 mpg (the maximum of the readout). As an experiment, when there were no other cars around, I compared coasting in gear with coasting in neutral, and I found that there was no difference.

Since the fuel injectors on modern cars are shut off when there is no application of the gas pedal while you are moving, there is no real difference in gas mileage between coasting in gear and coasting in neutral. The main difference is in the extra wear and tear on the friction surfaces in that expensive automatic transmission when someone decides to keep taking it out of gear and then putting it back into gear.

Trust me–when you get the repair bill for that transmission a couple of years after the warranty expires, you will wish that you had kept the car in gear instead of coasting in neutral to try to save money.

On my car and many, but not all, others the fuel it totally shut off when the car is in gear and going fast enough to turning the engine over as fast or faster than idle. That is free miles.