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Gas Mileage and Auto Transmission

Why do modern electronic automatic transmissions not slip into neutral when you lift your foot off the gas? Wouldn’t this save a little bit of gas? It could shift into what is the current normal mode as soon as you touch the brake for downhill engine braking.

I would think that the in-town EPA estimate could go up a hair which could make the sale of the car.

You definitely roll much further in neutral than in drive.

Actually, they save gas by staying in gear. Most modern engines shut off the gas when you lift off the accelerator until the engine speed is quite low. Shift into neutral, and the gas has to be used to keep the engine running.

The computers in newer cars shut off the fuel injectors when you lift your foot off the throttle going downhill or coming to a traffic light, as long as the transmission is in gear. If the transmission were in neutral the engine would need enough fuel to maintain idle speed, but in gear it needs no fuel at all, so you actually save gas by keeping the transmission in gear.

You may roll farther in neutral, but you burn more fuel.

That doesn’t seem to be what my mileage computer thinks. I do admit that it may not be that smart. It really notices if I shift into neutral for a mile.

I seem to be dumping a lot of energy into the engine (good for downhill braking) as the rpms are up a bit ~1500rpms. Doesn’t the fuel injector computer have to keep the cylinders warm? Perhaps not. If they did there is a minimum amount that it would have to inject in each cycle in order to A) burn and B) not create so much CO that the Cat couldn’t convert it. Wouldn’t this be more than the fuel I would need to maintain a 800rpm idle? I know that the butterfly (or whatever it is called these days) is closed but the engine must still suck a lot of air into the cylinders thereby needing some minimum amount of fuel. If there is a high vaccuum[SIC] in the intake, aren’t I dumping a lot of my velocity energy into the engine to make it turn this fast?

Does it open the butterfly for less pumping losses? If it does shut down the gas, does it also shut down the ignition? Is it OK for the cylinders etc. to cool down that quickly? I know that some engines can shut down whole cylinder groups when not needed but don’t they play games with the valves so that those cylinders are not such a drag?

I am just trying to understand why it behaves this way. I know that much older cars also dragged on the engine in the higher gears (3rd) but didn’t they also have a one way sprag clutch between the tranny and the torque converter?

Thanks for your time.


Wow, jmunn sure has a vivid imagination!! Why don’t you put a large electric fan on the rear of your car? That should give you more power and save fuel!!

LOL. Sorry, but as a mechanical engineer and a car enthusiast, your description is so funny!

Doesn’t the fuel injector computer have to keep the cylinders warm?

Believe me, the cylinders stay plenty warm when the fuel cut-out is used. The engine management computer will cut the fuel anytime your foot is off the gas pedal, and the RPMs are above idle. There is no appreciable air entering the cylinders when this happens. The throttle is closed, the IAC (idle air controller) is closed, and the rest is a closed vacuum system. No air means no fuel. The vacuum goes high, because the engine is trying to pull air in, but the throttle system is shut down, allowing none to pass. This creates a lot of energy that actually makes a resistance called engine braking. The car stays hooked to the transmission through a lock-up converter to use engine braking. The lock-up converter does not unlock until you either hit the brakes or slow down enough, generally below 30 MPH.

When the RPMs drop to a certain level, generally about 1000 RPMs, the torque converter unlocks, and the IAC will crack to allow some air in, and the fuel injectors turn back on. When you drop the transmission in neutral, the computer instantly has to crack the IAC, and turn on the injectors to keep the engine at idle. If your slowing down, this is fuel that is being burnt that could be saved by staying in gear. If your going down a long hill, your really wasting gas by dropping into neutral. Also, risking burn out on your brakes. This is so dangerous, it is illegal in many mountain states.

Don’t worry about the cylinders cooling down. The internal temperature of the cylinders varies quite a bit under normal conditions and is part of the design.
Combustion temperatures can exceed 2500 deg F, and air entering the cylinders generally are about 100 deg F. Under engine braking conditions, the cylinder temps actually stay more consistent, because that cool air charge is not happening. This is worrying about nothing. Also, don’t worry about wear. The oil pump and cooling system are still running, and providing the necessary protection for the moving parts of your engine.

Why do modern electronic automatic transmissions not slip into neutral when you lift your foot off the gas?

One good reason is you could not downshift to control your speed going down long steep hills without overheating your brakes and crashing. If automatic transmissions worked this way, nobody could ever drive through the mountains.

To Busted, I have a 96 Dodge van and you're right, the torque converter stays locked up until it slowes to a certain speed, however I had a 99 S-10 and the converter unlocked anytime I let up on the gas or if I touched the brake petal.  I think I like theDodge idea best.
To jmunn, I think your idea is just plain crazy!!

Older cars have never had a one-way clutch between the converter and the tranny. And I doubt very much if modern cars shut off the gas when one lets up on the accelerator.

mcparadice, you probably know more about this than I do, BUT I doubt very much that the gas is shut off when you let off the petal. I imagine the gas remains at an idle stage.

Elly is right. When you take your foot off the gas of a modern automobile the pulse width of the injectors is ramped back to idle width. The combination of the throttle position sensor signal and the sudden drop in manifold absolute pressure combined with the engine speed sensor and mass airflow sensor all combine to effect this.

If the injectors were shut down the ECU would record misfires. The CEL light would be ubiquotous (sp?) enough to make you oblivious to its presence.

The exceptions in the infamous 4/6/8 V8 engines where some cylinders are shut down to save fuel and in some hybrid setups.

I doubt very much if modern cars shut off the gas when one lets up on the accelerator.

Well some do and it becomes more every year. I suspect it is the majority of current models. There is no reason to use fuel under those conditions.

VW tells me that mine are shut off. Don’t you think the computer is smart enough to know the fuel is shut off and to ignore the misfire signal?

I wondered about this too. Since the ECU controls both the fuel delivery and the spark, it can easily disable them both at the same time - avoiding any chance of misfire.


Sorry EllyEllis, but many modern cars do just that, they zero out the injector timing. Why would you say they don’t?

Hey, texases, are you sure that modern engines shut off the fuel when you let off the accelerator?? This seems a little hard to believe.

well, joseph, I’ll take your word for it.

He’s right. Late-model cars from just about every manufacturer are programmed to shut off the fuel injectors when the vehicle is coasting in gear. There is no need for fuel under these conditions, so they don’t inject it. As the vehicle slows to lower speed the injectors are turned back on.

It’s more fuel efficient to keep the transmission in gear that it is to shift into neutral.

It’s not crazy, it’s smart. You don’t need fuel to keep the engine running under deceleration. Therefore, no fuel in injected. Aren’t computers wonderful?