Reading this question - Is engine braking harmful? - reminds me of a question I have always wanted to ask.

I used to own a Prius. When the Prius is cruising at low speed at a slight downward slope, the engine is frequently turned off spontaneously as well as not having any electric input into the drive motor. Therefore, the combustion drive is also disengaged.

My question is, why can't auto manufacturers transfer that technology/mechanism to non-hybrid combustion cars.

Nowadays I drive a manual shift. When I need the car to cruise at low speeds, I often step on the clutch to avoid the idling engine from dragging the speed of the car down. I also avoid accelerating the car too fast so that I could reduce the frequency of having to lift my foot off the pedal to get engine drag to slow the car down. So that while I can achieve 45 mpg, someone else driving the same car in a very rugged style would only get 38 mpg.

Whether automatic or manual transmission, why can't/don't manufacturers transfer that spontaneous transmission disengagement/re-engagement mechanism to non-hybrid cars? 45 vs 38 mpg is very significant fuel saving. Why can't car makers exploit that mechanism?

  • Why would idle drag your speed down (given proper gear choice)? I have 1986 old beat-up BMW with 425 000 km (265 000 mi) on the odometer, and it can run away from me on the parking lot in third gear on the idle alone (without touching accelerator at all).
    – theUg
    May 31, 2012 at 5:15
  • On a long country road drive travelling at 2-3 degrees down slope at 40 mph, the idling engine on highest gear drags the car down to 30 mph. If I accelerated it back to 40 mph and then freed the clutch, it would sustain 40 mph. May 31, 2012 at 5:36
  • First, remember, with throttle released, engine is not idling, FI completely shuts-off the fuel (unless, of course, the grade is long enough for the engine to reach idling speed and chug along, though if I can get to 20 mph in third on flat, why would not fifth push it on the downgrade?). Second, with motor disengaged (i.e. in the absence of engine braking), the vehicle should accelerate indefinitely (if slowly), not just maintain the velocity (theoretically, up until aerodynamic drag neutralizes the effects of gravity — I’ve experienced that effect when downhill skiing at high speeds).
    – theUg
    May 31, 2012 at 6:16
  • 1
    If your car cannot maintain a constant speed with light throttle pressure, you are in the wrong gear. When you lift off the pedal completely, the car will decelerate through engine braking - If the car automatically disconnected the engine you'd lose the engine braking! In fact, in some jurisdictions, it is illegal to drive with the car out of gear (or the clutch held depressed) as you are not considered to be in full control of the vehicle
    – Nick C
    May 31, 2012 at 9:17

3 Answers 3


I think your question may be misguided. The final effect you seem to be looking for is to be able to cruise at a specific speed while using the least amount of petrol, is that correct?

This is absolutely possible in modern internal combustion engine cars. You don't do it by turning the engine off and on again, or by depressing the clutch to let the car freewheel though.

You do it by choosing the right gear and the right engine speed - you should not be accelerating to a speed then disengaging the clutch to let the car slow down again. That would actually be less efficient.

In your specific instance of a gentle downhill slope, I think if you want an idle speed in top gear of more than 30mph you might just need to increase your engine idle speed - but this will also impact fuel economy.

Lastly, if fuel economy is your end goal, have a read of this question which covers the topic nicely.

  • Turning the engine off and on will always achieve the highest possible efficiency as long as you don't use the starter to turn it back on (for instance, if you pop the clutch to restart the engine). This is the only way you can achieve zero "engine drag" and zero fuel usage simultaneously. I've measured mileage up to 52mpg on a car that "normally" gets about 38mpg using this method under moderately hilly highway conditions. Jun 3, 2012 at 6:17
  • Your statement is not true. I am cruising down hill at a slight slope of 2 deg. My Honda Civic runs at 40 mph on the highest (4th) gear. I lift my foot off the gas pedal, the idling engine drags it down to 30 mph. I step on the clutch to free the clutch, the car gradually slips back into 40 mph. Is that not called engine drag slowing down the car by an idling engine? Jun 3, 2012 at 21:23

It would depend.

On automatics this is sort of the case. The engine is not directly connected to the transmission and they "coast" a bit more than manuals left in gear. Unless they are in downhill mode, which keeps the converter locked up, and functions similarly to a manual.

I find that one of the benefits of a manual transmission is that the choice is up to me. It depends a lot on context. Sometimes I just want to save brakes and gas at the same time, and I let the engine do the braking coming to a long red light. Otherwise, I might put it in neutral and coast because I know it will turn green soon. If the car decided to place itself in neutral at every opportunity, it might not always be the most efficient decision. For example, if I knew I had to stop ahead, engine braking would save gas where as disengaging the transmission while remaining in neutral would waste gas (unless the engine is turned off).

On a related note, There are some manual transmission cars that do turn themselves off when the car is stopped and the transmission is in neutral. Upon initial depressing of the clutch, the car quickly starts up again. The BMW 118d, I believe, does this.

I don't like this from a technical point of view. Engine starting is one of hardest things an engine has to suffer. A way to relieve some of this stress is to have an oil pressure reservoir of some sort. For all I know the BMW example I mentioned might have this.

  • For the Prius, if you lift your foot off the pedal, magnetic braking takes effect. However, if you depress the pedal a little to keep it cruising, you can achieve a state where there is neither electric input to the motors nor combustion engine activity. Therefore, the question - why won't auto makers transfer that technology to non-hybrid cars? May 31, 2012 at 4:00
  • What you are referring is so-called start-stop technology. It is a lot more prevalent in Europe and Asia than in U.S. Restart of the engine is not bad in these conditions, cause the engine stays warm, and oil is at top lubricity. Starting is stressful most on the cold engine. As for ease of use, start-stop with manual seems a lot more intuitive with manual (where you do the thing you do anyway — stop and pop it in neutral), and less so with automatic.
    – theUg
    May 31, 2012 at 5:24

This is an attempt to answer my own question. This what I perceive but it may not be true. I wish someone could verify or invalidate this anecdotal observation of mine.

I was driving across the whole stretch of I-80 in Nebraska. Relatively flat terrain with some infrequent undulations. Ideal for hypermiling. The speed limit was 75 but I was driving at 55 mph. And the gas prices were $3.299.

But I managed only to attain 40 mpg across Nebraska. In fact, it did not make any difference whether I drove at 40 mph or 60 mph. It was still 40 mpg. Though driving at 75 mph dropped it down to 30 mpg.

The Honda Civic/Accord has a sticky gas pedal, someone told me. When you depress the gas pedal it will reach a point of impedance to your foot pressure. After which, you would have to press harder to inject more fuel into the engine.

Here was my experiment:
When I depress the gas pedal until the point of stickiness, I have anecdotal observation from the sound of the engine, that the engine is still idling at minimum fuel intake.

Until my recent long distance drive, I had tried depressing the clutch to avoid engine drag to allow me to cruise at 40 mph. Which does get me 40 - 42 mpg.

However, just for the past week, I decided to try never to disengage the clutch but to cruise on the 4th gear with the gas pedal depressed to the point of stickiness. There is some magic going on with the Honda sticky gas pedal. After 300 miles of driving that way over the past week, mostly cruising between 40 - 60 mph, I am getting 55 mpg. And, I was not revving up to 60 mph to cruise down to 40 mph. I was driving at constant speed depending on the speed limit of the road.

I am getting 55 mpg, without performing weird tricks like cruising to the traffic lights at 5 mph. I merely ensured I do not accelerate too quickly and kept a 500 ft separation from the car in front of me. Of course, Colorado drivers were showing their anger at me by deliberately zipping past me with their truck engine revved up or beeping at me for traveling at 50 mph on a 45 mph speed limit. I find it difficult to understand why they race to the red light at 40-50 mph to wait for it to turn green when you could cruise there? Nobody does that to you in the Puget sound peninsula.

My theory is, the sticky point on the Honda gas pedal is somehow connected to freeing the clutch to allow the car to cruise without being dragged down by an idling engine. If that is true, Honda is brilliant.

If not, what else explanation could there be for my having achieved 55 mpg, cruising 40-60 mph without needing to step on my clutch? The point is - if I lifted my foot off the gas pedal completely, engine drag would slow the car down to 30 mph. So Honda's sticky gas pedal is a brilliant piece of engineering that I do not understand. And my car is not even a hybrid.

Perhaps, Honda has already done what I asked in this question and I had not been aware of it.

  • When I wrote 4th gear - I should actually mean 5th gear. And yes, still true, without much effort, in the summer, my Honda could gets me 52 mpg as long as I don't drive faster than 55mph. And they won't let me use the HOV lane even though I get better mpg than Prius. Oct 4, 2012 at 10:45

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