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When spotting a red light or similar, I often release the gas pedal, gear down and let the engine decelerate instead of pressing the brake pedal. I was taught that this was more energy efficient for some reason, but I never really understood why. Is thIs true?

Oddthinking
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Benjamin Lindqvist
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    Might be worth reading http://mechanics.stackexchange.com/questions/1210/does-downshifting-engine-braking-cause-extra-wear-and-tear – Cornelius Jun 14 '14 at 11:38
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    I've removed two theoretical answers, including the accepted one. Please remember that we don't allow answers based on mere theoretical models - you *have to* link to evidence. – Sklivvz Jun 16 '14 at 22:53
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    I am familiar with a related claim (using the engine to slow-down saves wear on the brakes and reduces the chance of overheating) and a related counter-claim (that advice was from when brakes were unreliable; now brakes are reliable and the gearbox is expensive, so the advice is out of date and primarily rely on brakes). There is a side-issue of regenerative braking in electric cars. However, I haven't heard of using the engine to be more efficient as claimed here. Does anyone have a notability reference? – Oddthinking Jun 17 '14 at 02:54
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    The only efficiency claim is one that I've anecdotally tested with the trip computer on the car. Downshifting to higher revs, and coasting, causes the instant fuel consumption to report 0.0l/h. But braking only reports 1.1l/h (l/h is used over l/100km when not throttling). So hardly a massive difference in economy, and personally not worth the extra wear on the gearbox. But also sounds very, very cool if you have a large capacity engine or a nice exhaust. – Mark Henderson Jun 17 '14 at 03:37
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    I think there was even a mythbuster episode where they tested all of this quite precisely. Depending on the conditions I would say that I personally save 10-15% with this style, as this is about the only major difference in me and my wifes driving style. – PlasmaHH Jun 18 '14 at 10:03
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    I think the problem is that this question is a bit too vague - the type of vehicle being driven and a whole lot of difficult to track variables come into play. Some examples: A hybrid/electric car may benefit more from the use of the brake, as this causes the battery to be charged. An older carburetor style car as previously stated may offer no efficiency savings. Then their is the energy cost of replacing brake pads as well as the potential wear and tear on the engine. Finally you may need to consider safety - you probably have more and better control of your deceleration with the brake. – Chris Noldus Feb 10 '15 at 03:04
  • As someone who was taught to drive in the UK in the 80s by a driving instructor I seem to remember the reason to gear down was more about the relative inefficiency of the brakes rather than being more energy efficient. I didn't sit a test until post 2000 and the next instructor advised it was no longer advice in modern cars. – The Wandering Dev Manager Sep 04 '15 at 08:06
  • I had one driving instructor tell me this; then another, in a more modern car which kept stalling when I used the same engine breaking tricks that worked well before, say that it was a myth/legacy from the bad old days of easily-worn-out non-ABS brakes. I get the impression there are similar pros and cons of each and which is best varies depending on the driver and on the car. – user56reinstatemonica8 Feb 03 '16 at 17:59
  • Is there a notable claim that gearing down increases efficiency? All that's here is "I was taught this for some reason." I've always understood that the benefit is reduced wear on the brakes, which the instructor may have misunderstood. – Mark Feb 04 '16 at 11:06
  • Related: Driving down a steep road, should we use a lower gear or ride the brakes? Mountain roads around here sometimes have signs: "Use Lower Gear". – GEdgar Feb 04 '16 at 15:14
  • @GEdgar: On long mountain grades, the brakes can become hot, causing them to lose effectiveness. – jamesqf Feb 04 '16 at 18:52

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I've worked as eco-driving instructor for TCS (Touring Club Suisse), explaining people how to be the most energy efficient while driving a car. I've also spend a lot of time driving a car where you can see the "instantaneous fuel consumption", and I've seen all I write in real life. I've also drived a lot of automatic car, includind Toyota Hybrids with Eco Mode. All the following assumes you drive a car with an electronic injector as opposed to a carburator (this is true for most cars that are less than 20 years old).

The basic rule I teach is: lower revs per minute -> lower fuel per mile. It's not always true, but it's mostly true. Example experimental demonstration.

But there is more. If you coast, the motor uses fuel to stay at idle speed, but when you have the gear engaged, the injector uses an interuption of fuel injection during deceleration. What is important is that the interuption only occures when the rpm is over a minimum (to avoid motor stall).

Another things to look at, is what automatic cars do. Automatic cars are designed to spare the engine and save up fuel. When you are driving downhill, your automatic car will automatically choose the highest possible gear. If you stop pushing the gaz pedal, your automatic car will automatically choose the highest possible gear. And this is what a Toyota Hybrid's Eco Mode will advice you to do. This means that the "use the highest gear possible" is not just an idea of gouvernement mandated experts. It's what the engineers that designed the car want.

But the conclusion is that, energy fuel consumption wise, braking and gearing down both use no fuel. (So long you don't feel your car giggling because rpm are too low)

At the TCS, we advised people not to gear down, because engine braking does not benefit from the anti-lock braking system (ABS), and because it's better to wear down brake pads than the engine. We have been told that the TCS engineers did the math, but I could not find a reference for those calculations.

But this was not true 30 years ago, when cars had carburators. With a carburator, gearing down uses more fuel than braking. But, at the same way, without an electronic injector, driving at low rpm was bad for the engine (because the air-fuel ratio was not allways optimal). And this is probably why your parents' driving instructor taught them to drive that way, and why they tell you to do it.

Spoutnik16
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  • I'm sorry for having no english source. This is because I just don't know the words on english for fuel injection interruption (which probably has an english word for)(or because nobody cares about it in the english world) – Spoutnik16 Feb 04 '16 at 08:37
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    Great answer, +1, and the English was mostly very good. I've suggested a couple of small corrections, and a reference for one of your points which didn't have a reference. I think there is a specific technical term for "fuel injection interruption", but I can't think what it is right now - but how you've phrased it is comprehensible – user56reinstatemonica8 Feb 04 '16 at 09:26
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    I'm confused... if gearing down used more fuel, why would the driving instructors be recommending it? – Sean Duggan Feb 04 '16 at 12:33
  • @SeanDuggan : edited to clarify it. – Spoutnik16 Feb 04 '16 at 13:05
  • @Sean Duggan: Because gearing down gives you better control. – jamesqf Feb 04 '16 at 18:54