Best of Deals Car Reviews Repair Shops Cars A-Z Radio Show

Idle in gear or in neutral

A question during the show this week (1/22/2011) was whether it used more gasoline to idle in gear or in neutral with an automatic transmission. The answer was it makes little difference.

I contend the engine uses more fuel idling in neutral. The reason is the increased level of partial vacuum within the intake manifold means more air and fuel will be drawn into the engine. When the vehicle is in gear, the engine idles slower, less vacuum, less fuel used.

Seems like your going to burn more in gear as its under a load or working

Peacefrog - I agree with your thinking. However, since the butterfly valve stays stationary in the throttle body, the only way to get more fuel through it is with a higher vacuum. The extra fuel is used by having the engine turning faster and the additional friction. The slowing of the engine by the clutch reduces the vacuum so the overall fuel use is less.

You’re overthinking the problem. The bottom line is that the ECU bumps the idle up when in gear to overcome the added resistance of the torque converter fluid. That uses a bit more gas. The TC converts the extra energy coming from the engine into heat that’s dissipated by the transmission’s cooling system.

More load will always take more fuel.

"the butterfly valve stays stationary"
This may be true in some cases, but it isn’t in most of them. My cars like most recent cars are throttle by wire. Idle is controlled by the throttle, and the the throttle plate does move ever so slightly to keep the same idle speed when shifted into gear.
On older cars, the throttle plate would stay at the stop position, but the idle air valve would open more, which would be the same result and the throttle opening slightly.

All that said, the difference is very minor. I keep it in gear.

“The answer was it makes little difference”

And that is the correct answer. I doubt unless you had some very sensitive testing equipment you would be able to even tell the difference. Their recommendation to shut the car off is the best scenario, but even then you would be hard pressed to tell.

Yes, very very little difference…but I can say for certain it takes a few Watts more to enagage the maintain the pressure on the forward clutch and torque the driveline up than it does to leave it in a neutral state…in physics, nothing is free.

It is like “does it use more fuel to drive with the lights on”? You have to answer yes as there is that “no free lunch” aspect. I have long proposed that we set a standard for when an activity actually falls into the “uses more fuel” or “uses less fuel” area.In short, let’s insert a pratical nature in the answer/question and not get hung up on a strict technical interpetation. This thing could be never ending as someone can say “if 200 million cars all achieved a .1 mpg increase do you know how much gas that would save”? We must try and stay pratical.

It sounds to me like you’re thinking of this as if it has a carb - the amount of fuel fired through the injectors is determined by the ECU/PCM. And as mountainbike correctly noted below, the in gear automatic increases load.

In any case - as also noted - the best answer is that it doesn’t make enough difference to matter to anything.