…or not to coast — that is the question.
From a safety (and brake wear) standpoint, I agree that it’s a bad idea to coast down a hill — you should use the engine to help brake the car. But I often coast pulling up to a stop sign or red light. I can whip it back into gear fast enough if I have to, and if I’m already in gear, it’s probably the wrong one for a rapid acceleration, unless I’ve been downshifting.
As for fuel economy, I’ll accept his argument for fuel injected cars, but I’ll bet that coasting saves gas over having the transmission engaged for a carburetted vehicle (do they even make them any more, though?).
[Update a few minutes later]
I guess not, if Wikipedia is to be trusted:
Carburetors were the usual fuel delivery method for most U.S. made gasoline-fueled engines up until the late 1980s, when fuel injection became the preferred method of automotive fuel delivery. In the U.S. market, the last carbureted cars were:
* 1990 (General public) : Oldsmobile Custom Cruiser, Buick Estate Wagon, and Subaru Justy
* 1991 (Police) : Ford Crown Victoria Police Interceptor with the 5.8 L (351 cu in) engine.
* 1991 (SUV) : Jeep Grand Wagoneer
* 1994 (Light truck) : Isuzu[5]Elsewhere, certain Lada cars used carburetors until 2006. A majority of motorcycles still use carburetors due to lower cost and throttle response problems with early injection setups, but as of 2005 many new models are now being introduced with fuel injection. Carburetors are still found in small engines and in older or specialized automobiles, such as those designed for stock car racing. In such applications, carburetors reliably supply very high volumes of fuel at full load and are easy to set restrictions on to give even, fair racing.
So there you go.
I can whip it back into gear fast enough if I have to, and if I’m already in gear, it’s probably the wrong one for a rapid acceleration, unless I’ve been downshifting.
An American with stick shift? Not that I know how to drive a car…
An American with stick shift?
Yes, I hate automatics. But it’s getting harder to find cars with a stick over here. It’s almost impossible to rent them. The last time I did so was in Hawaii.
I can’t believe I have to tell a rocket scientist this. 😉
Coasting can save gas if the transmission would otherwise braking, by taking full advantage of gravity to accelerate the car. It has nothing to do with carburetors.
Gravity!
You may exceed the speed limit, though, so the key is to start down the hill at a low enough speed.
Yours,
Tom
I’m quite aware of gravity, thanks. If you make the engine spin with a carburetor, it’s going to suck and burn some fuel, even if there’s no load (I think). As the PM article points out, this doesn’t happen with electronic fuel injection, because the injectors don’t fire if there’s no load.
I just bought my first automatic transmission car due to difficulty in finding a manual here in the US. My theory is that too many people want to talk on cell phones and manual transmissioned-cars make that kinda hard. My original plan was to stay with manual until my kids learned to drive but, oh well.
The part about the fuel injection shutting down really caught me by surprise. I figured they would loaf along at idle speed, thinking that if there was no gas flowing, a slowdown would be more obvious.
Just goes to show that it’s not what you don’t know that burns you, it’s what you think you know…
On any car with a MPG gauge or display (a ScanGaugeII will also show the actual fuel flow) you will see the MPG max out when going downhill in a lower gear or any deceleration with the transmission in gear and after the engine has warmed up. On coming down out of the Sierras, I have engine braked long enough in my 00 Passat (stick) that the thermostat closes and the coolant temp starts dropping. My 06 Prius injects enough fuel now and then to keep the catalytic converters from getting too cold and increasing emissions.
VW still sells several cars with sticks. Finding them may be a problem…
Echo others, I just got my first automatic car that I drive regularly (wife has had them for some time). I did at least hand down my manual to one of my daughters, so she will learn to drive with a stick.
My theory is that too many people want to talk on cell phones
Eh, I got bluetooth in the new car. I have a hard time not getting bored. Audiobooks help past the time.
I think the carburetor idle circuit will still drop full into the engine even while coasting, too. I know the metering jets will be closed under high vacuum like that so the primaries will certainly not be doing anything. But I don’t believe there is vacuum actuated valve that closes the idle circuit off while coasting in gear like that. At least not on the Holley carbs I’ve worked.
I do know that my Subaru’s idle enrichment map is not active above 1000 rpm and 0% throttle. I actually have a Tactrix ECU scan tool to monitor my engine in real time. Now I’m curious to fire up a log on my injector voltage to see this in real time on my car.
Another thing that I know will affect your vehicle mileage quite a bit is a dirty MAF sensor. The MAF sensor works by heating up a platinum wire to a specific temperature. As the engine runs it draws cold air from outside, through the air cleaner, and then into the MAF sensor. Cold air passing over the hot platinum wire in the MAF sensor cools it down. The higher the RPM’s, the more air gets pulled into the engine, and the colder the MAF wire gets. The ECU counteracts the cooling by increasing the voltage to the wire to keep it at a specific temperature. The ECU determines the amount of air that is going into the engine based upon the increase in voltage to the MAF wire. It is important to know how much air is going into the engine because that determines how much fuel needs to be pumped into the cylinders by the injectors.
If the MAF wire is covered in dirt and grim then it cannot radiate heat away properly and will provide a false reading to the ECU. The thing is, the ECU is actually smart enough to know something isn’t right because the readings from the O2 sensor will not match up with expected values in the O2 scaling tables. So, the ECU will invoke air fuel compensations to adjust for the inaccurate MAF readings. Only problem is that these compensations are just estimates and it generally will over compensate (making the engine run rich) rather than under compensate. On my car I’ve seen the A/F compensation during closed loop run at around 2-3% enrichment. I’d like it to be 0% but 2% is not too bad. It’s when the enrichment compensation gets up over 5% that you have to wonder just how much fuel the engine is needlessly piddling away. Not only does this ruin your fuel economy but it also robs you of power. Clean out a dirty MAF sensor and you can get a good 2-3 HP back and easily restore 1-2 mpg to your economy.
They actually make a MAF sensor cleaner in a spray can that you can pick up from the parts store. The sensor is usually pretty easy to get at. Just unplug the sensor and loosen 2 screws and pull it up and out. NEVER touch the platinum wire. Spray it down with a electronic parts solvent or MAF sensor cleaner. Oh, and avoid getting a dirty MAF in the first place by performing regular maintenance of your air filter.
I drive a stick and have for nearly three years. It’s my first — before that I took the bus for four years — in Orlando, Florida, so I was pretty desperate to get a car… And before that I’d always driven automatics. It used to be I vowed never to drive a stick because it was just “too hard” but now I love it.
I live in a town with a lot of hills now (in Virginia) and I do the coasting thing down the hill around the corner from where I live. If I don’t I end up using the brakes anyway to keep the car from exceeding the 25 mile per hour speed limit on that street. So I might as well put it in neutral. It’s just about a block or so anyway.
My theory is that too many people want to talk on cell phones and manual transmissioned-cars make that kinda hard.
No, it’s just that most people never learned to drive with a manual transmission. This trend predates the cell phone.
You can still talk on your cell phone and drive a stick. You just get a Bluetooth, or an earbud for your phone. And you’re not always shifting gears so you’ll often have your hands free.
Yeah I’ve done it. But I don’t like to — I can’t hear the phone very well in the car anyway, and I find it distracting. Also I’ve never liked talking on the phone. I once had a friend of mine end our friendship because I got tired of her long evening phone calls where she’d describe the entire plots of that day’s soap opera episodes. I could never get a word in edgewise. To this day I hate soap operas.
I was in a Triumph motorcycle dealership a few weeks ago. The new Bonneville is a retro design that looks like the bike of the 50’s. I spent some time examining the carburetors before realizing they are actually fuel injection units that are designed to look just like carburetors.
Amazing. And, yes, I also buy manual transmission cars.
> I’m quite aware of gravity, thanks.
OK. If you work to conserve the energy added to the vehicle by the acceleration due to gravity by coasting, rather than engine braking, doesn’t that save gas? It has nothing to do with the idle.
Similarly, don’t you get better mileage with the wind at your back, and less when you drive against it?
Yours,
Tom
I found the bit about fuel injectors simply shutting down to be quite fascinating, and, in retrospect, obvious. What engineer would fail to put that command in the microode?
I think the distinction between a manual and an automatic has faded considerably. They used to be godawful clunkmonsters, 3-speed even in a 4-cylinder car (!) and huge gas hogs. But now…they’re pretty damn smart, and they often the same or almost the same MPG. I still prefer to shift the gears myself, since after 31 years it’s about as natural as walking, and I’m still annoyed that the transmission is blind and dumb, and does things like burn off 5 mL of gas unnecessarily shifting into a higher gear just as the car gets to the bottom of the hill, so it will have to shift back down again 10 seonds later.
But I bet soon enough they’ll have eyes and radar and little whiskers on the beasts, and they’ll be able to shift even smarter than humans anyway. Face it, we’re obsolete.
If you work to conserve the energy added to the vehicle by the acceleration due to gravity by coasting, rather than engine braking, doesn’t that save gas?
How do you conserve the energy? Unless you have a flywheel to put it into, you’ll have zero energy at the bottom of the hill, regardless of whether you use the brakes or the engine to control your speed. If there is no load on the engine (you’re using it to brake), the fuel injectors will not feed any gas to it, but if it’s idling in neutral, they will.
I’ve mostly driven manual transmissions, from the three-on-the-tree Ford pickup from high school, the MGB for my first years of college, the gray market BMW 320e, and currently my VW New Beetle Cabriolet. Wouldn’t drive anything but a manual, though I don’t particularly make an effort for rent-a-cars. I just find that manuals give me a better “feel” for the car, and better engages me in the driving environment.
One thing you’ll never find me doing is talking on the phone while driving. People that do so and are not in the far right lane just drive me nuts, because you can tell they just aren’t paying attention. They’re like rocks in a stream, with the traffic flowing by on both sides.
As for the coasting, I do so all the time. I’ve got a good feel for the car at this point and use both engine drag and coasting frequently as parts of my driving strategy. I’m with Rand on the rapid response. Automatics just have a lag when you try to get them to respond quickly, whereas I can exactly pinpoint my acceleration from knowledge of where the clutch engages. As for sharp cornering, I was taught that you hit the acceleration halfway through a curve. Before that point the clutch is in, effectively in neutral, while I decide on first or second gear for the acceleration out of the turn. FWIW, my first set of brakes on the Bug lasted for 100,000 miles, and one pair is still on the car with some 30% left. How much money has that saved me? Economical operation of a vehicle is about more than just squeezing out MPG. (Sort of like how spaceflight is about so much more than just the delta-Vs between two points in space, just to give things a transterrestrial aspect)
It does sound like the article was focused on automatic transmissions. I do have to wonder about the suggestion at the end to turn the engine off at a red light. If being in neutral is dangerous, wouldn’t being inert be much more so?
Is it that hard to find manuals? I found many used manuals on Autotrader and Galpin Mazda had at least one standard 3 in stock. I was looking at small, relatively sporty cars. Perhaps standards are much rarer in other classes.
Golly, there is so much misinformation out there, I don’t know where to begin. And I think that some of this misinformation comes from my Scan Gauge.
I rented a Renault “Scenic” in Germany that had a display mode for “litres/hour”, and when you coasted in gear (a manual — I think they only rent automatics to a few of us Americanischers who are complete Wieners), the display showed zero. When you took it out of gear and coasted in neutral, the injectors came back on to restart the motor and you got the idle fuel flow, which if I remember, was around 1 litre/hour (on a 1.6 L 4-cylinder — a “big” engine in Europe).
On my small fleet of U.S. cars, the Scan Gauge never shows a complete gas shutoff when coasting in gear. I guess emission laws are actually much stricter in the U.S., and keeping the combustion chamber warm may reduce those emissions. I also read in Ford World magazine that the Ford Hybrid system on the Escape never completely shuts of the engine in electric mode for that reason.
As to the Scan Gauge, not sure how much to trust it as each of three cars I have tried it on (Camry, Taurus, Sienna) needs a different “fudge factor” correction entered into the tank-fill reset so that the gas the gauge thinks you used matches the tank fill, although you can never match the tank fill because it is hard to fill the tank exactly to the same level each time.
So I believe the Scan Gauge to an extent because I have nothing else to go on. From Scan Gauge feedback, that thing Toyota is shilling in Sweden about “drive as if you have a glass of water balanced on the hood” is utter and complete nonsense. If anything, in city traffic you want to keep your speed up because gas mileage increases with increased average speed, that is, until you really start rushing and end put leaning on the brakes a lot for the inevitable stops and speed reductions.
In Scan Gauge terms, I get better trip gas mileage sometimes when I am a tad rushed and tip in a little more throttle to accelerate faster. The thing is, it is not so much this business of avoiding “jack rabbit starts” as avoiding “hare-brained stops.” I try to think “accelerate and then lift (foot from throttle pedal) and coast, coast, coast.”
But then again, “everyone else” seems to be “faster off the line” than I am, even when I am driving according to my normal habits and am not distracted by the Scan Gauge. So to the driver whose modes are either “give ‘er the gas” or “hit the brakes”, perhaps the “glass of water” advice makes some sense. But the “hypermiling” advice of “think there is an egg between your foot in the gas” is complete bosh.
In fact, the best city-mode trip gas mileage I read on the Scan Gauge is with a kind of “pulse-glide” driving where I alternate bursts of (what is for me) brisk accceleration followed by coasting as much as possible, squeeze in brake in advance of red lights and if I am lucky, I am rolling into the green.
As to coasting in gear using less gas than neutral, its not the MPG you want to look at but the gallons/hour mode of the Scan gauge, and for the Taurus at least, neutral uses less gas than coasting. The Camry, however, seems to do better “pulse and glide” (alternating between accel and coast — the engine always running and in gear — only for city driving — doesn’t seem to help on the highway). The transmission seems to “freewheel” better than on the Taurus.
One place you may want to try neutral (again, the safety issue if someone is behind you and you need to boogie) is if you are stuck at a long traffic light or at a train crossing. I tried switching the engine off, but there seems to be a gas penalty, not so much of the engine start but the “rewarmup” cycle after the engine is running again. On the other hand, idling in neutral shows lower gas flow than idling in gear, and the effect is more pronounced in the Taurus, that maintains tighter control on idle RPM’s than the Camry.
One of the advantage of hybrids (even “mild hybrids”, with very small batteries or an ultracapacitor) is the ability to truly turn off the engine when its power is not needed. If such a vehicle were programmed to turn off the engine in neutral then coasting would save fuel.
(I don’t think the Prius can turn off the engine above a certain speed, due to the details of the parallel hybrid transmission.)
Re: Paul D:
I drive a prius. the engine can and will be turned off at any speed if you don’t need it, such as when stopped, coasting, braking or going downhill. That is one of the tricks to get better mileage.
If you step completely off the gas, the engine goes off and you get some regenerative braking to simulate the “motor braking” people are used to.
But if you put just a little bit of pressure on the gas pedal, you are basically idling. You can see on the energy flow display that the motor is off and there is no energy flowing from the engine to the weels.
So the trick is to use this whenever possible. Not so much different from a stick shift, I guess. The advantage over going into neutral is that power is immediately available if you need it.
I think normal automatic transmissions are horrible. But the electric CVT of the prius is just great. You never have any delay between stepping on the gas and accelerating, and you always have the right gear. In fact, the prius accelerates pretty quickly for an eco car. I would love to drive a hybrid that was optimized for performance.
There are a lot of factors, I’m sure. Laziness in driving, cell phones, and improved technology in automatics are some cited here. The article is a bit old, but it says that 6% of cars sold in the US will be manual in 2012. I’ve seen that value quoted elsewhere, but they’re probably just repeating this one:
http://www.autoblog.com/2004/09/13/manual-transmission-may-become-extinct/
> Unless you have a flywheel to put it into, you’ll have zero energy at the bottom of the hill, regardless of whether you use the brakes or the engine to control your speed
You don’t control your speed using either the brakes or the engine. All forms of braking are the enemy of efficiency. You coast and start using the engine as you go up the next hill.
Yours,
Tom
All BMW motorcycles designed since the K100 series in 1984 have had fuel injection, and ABS brakes have been available on them since 1987 I think. Both features have been rare to non-existance on other brands of motorcycle until very recently.
My 1986 K100RT had a noticeable surge as you decelerated through 1200 RPM and the injectors cut back in. My current bike, a 1995 R1100RT has the cut off point at 1800 RPM and it’s much less obvious.
The new generation R-series twin cylinder bikes launched in 1994 and 1995 also introduced catalytic convertors to BMW motorcycles. I think that’s still extremely rare on other motorcycles today. Mine doesn’t have one as NZ still had leaded fuel (which would kill the catalyst) in 1995, but the later models sold here do.
When I was younger and in the military I had a manual toyota pickup that I drove all over the western US. On those huge downslopes I just shut the engine down entirely and coast, keeping the radar detector on to spot cops. Sure the brakes would get a little hot and wear out quicker, but they are easy and cheap to change. Going downhill for 10-20 miles at a time with the engine off would boost the trip milage significantly. Compression start at the bottom worked just fine. I had this jug of dissolved catalytic metals feeding an air line into the PCV hose that caused my catalytic converting to happen inside the engine too, so I didn’t have to worry about the converter cooling down. This boosted the pickups milage from 22 mpg to 27mpg.
I drive a 1988 Daihatsu Charade. 5-speed (2 overdrive), 993cc, 37 kw, and in city driving, 6.5L/100 km. On the highway, about 4.8 – call it 50mpg at 80-100 km/h (50-65 mph)
And when it finally wears out – doesn’t look like it will anytime soon – , if they still made them, I’d buy another one new. Fortunately a lot of sub-compacts are converging to the same formula, though with EFI not carb. Most don’t have 4-wheel discs though.
All BMW motorcycles designed since the K100 series in 1984 have had fuel injection, and ABS brakes have been available on them since 1987 I think. Both features have been rare to non-existance on other brands of motorcycle until very recently.
My 1986 K100RT had a noticeable surge as you decelerated through 1200 RPM and the injectors cut back in. My current bike, a 1995 R1100RT has the cut off point at 1800 RPM and it’s much less obvious.
The new generation R-series twin cylinder bikes launched in 1994 and 1995 also introduced catalytic convertors to BMW motorcycles. I think that’s still extremely rare on other motorcycles today. Mine doesn’t have one as NZ still had leaded fuel (which would kill the catalyst) in 1995, but the later models sold here do.
Bruce-
I think you may need to get out more often.
The F-series (which used to refer to a single-cylinder Rotax-powered Aprilia with a BMW logo on it, but now refers to a parallel-twin Rotax-powered BMW), through 1999, ran carburetors. They didn’t switch the F-series over to Injection until 2000, which, IMHO, ruined the model. The carb’ed Funduro models were much more responsive and fun to ride.
And, while catalytic converters may have been added along with an oil cooler to the R-series in 1994, I would hardly call the original generation of Oilhead engines the “new generation”. The R-series engine has been updated at least twice since 1994, with the addition of a second spark plug in 2004, a remodel of the head in ’07, and a complete redesign of the engine to DOHC in ’09, with changes rolling to the ’10 and ’11 model year bikes.
If it wasn’t for the increased weight and heat retention, and the fact that I don’t live in Kalifornia, my ’99 R1100 might still have its catalytic converter installed. 😉
That said, I haven’t noticed one way or the other if the engine cuts fuel upon deceleration or engine braking, as I rarely let the RPMs get low enough on engine braking to notice. Once they get much lower than about 2000 RPM, I’m back into the clutch, unless I’m performing really low-speed maneuvers with limited-if-any throttle.
Paul Milenkovic: Your technique sounds like the counterintuitive airplane flight plan for max economy, discovered by (IIRC) Southwest Airlines years ago.
Basically, it’s climb as fast as possible to max altitude, then coast, bleeding off altitude a little at a time to the destination. The engines loaf along at minimum power for most of the flight, which saves enough fuel to offset the high-power climb.
Airliners can’t actually use the technique, because they have to stick to airways and ATC-commanded altitudes, but even an approximation saves some fuel — read Captain Dave (FL390) on the subject of fuel calculations.
It would never have occurred to me that a parallel technique might work in a car.
Regards,
Ric
As you may know, Virginia is the only state that bans the use and sale of detectors. There is no evidence that the detector ban increases highway safety. Our nation’s fatality rates have fallen consistently for almost two decades. Virginia’s fatality rate has also fallen, but not any more dramatically than it has nationwide. Research has even shown that radar detector owners have a lower accident rate than motorists who do not own a detector.
Maintaining the ban is not in the best interest of Virginians or visitors to the state. I know and know of people that will not drive in Virginia due to this ban. Unjust enforcement practices are not unheard of, and radar detectors can keep safe motorists from being exploited by abusive speed traps. Likewise, the ban has a negative impact on Virginia’s business community. Electronic distributors lose business to neighboring states and Virginia misses out on valuable sales tax revenue.
Radar detector bans do not work. Research and experience show that radar detector bans do not result in lower accident rates, improved speed-limit compliance or reduce auto insurance expenditures.
• The Virginia radar detector ban is difficult and expensive to enforce. The Virginia ban diverts precious law enforcement resources from more important duties.
• Radar detectors are legal in the rest of the nation, in all 49 other states. In fact, the first state to test a radar detector ban, Connecticut, repealed the law – it ruled the law was ineffective and unfair. It is time for our Virginia to join the rest of the nation.
• It has never been shown that radar detectors cause accidents or even encourage motorists to drive faster than they would otherwise. The Yankelovich – Clancy – Shulman Radar Detector Study conducted in 1987, showed that radar detector users drove an average of 34% further between accidents (233,933 miles versus 174,554 miles) than non radar detector users. The study also showed that they have much higher seat belt use compliance. If drivers with radar detectors have fewer accidents, it follows that they have reduced insurance costs – it is counterproductive to ban radar detectors.
• In a similar study performed in Great Britain by MORI in 2001 the summary reports that “Users (of radar detectors) appear to travel 50% further between accidents than non-users. In this survey the users interviewed traveling on average 217,353 miles between accidents compared to 143,401 miles between accidents of those non-users randomly drawn from the general public.” The MORI study also reported “Three quarters agree, perhaps unsurprisingly, that since purchasing a radar detector they have become more conscious about keeping to the speed limit…” and “Three in five detector users claim to have become a safer driver since purchasing a detector.”
• Modern radar detectors play a significant role in preventing accidents and laying the technology foundation for the Safety Warning System® (SWS). Radar detectors with SWS alert motorists to oncoming emergency vehicles, potential road hazards, and unusual traffic conditions. There are more than 10 million radar detectors with SWS in use nationwide. The federal government has earmarked $2.1 million for further study of the SWS over a three-year period of time. The U.S. Department of Transportation is administering grants to state and local governments to purchase the SWS system and study its effectiveness (for example, in the form of SWS transmitters for school buses and emergency vehicles). The drivers of Virginia deserve the right to the important safety benefits that SWS delivers.
*** A small surcharge($5-$10) or tax(2%-3%) could be added to the price of the device to make-up for any possible loss of revenue from reduced number of speeding tickets and the loss of tickets written for radar detectors.***
Please sign this petition and help repeal this ban and give drivers in Virginia the freedom to know if they are under surveillance and to use their property legally:
http://www.stoptheban.org
http://www.thepetitionsite.com/1/repeal-the-virginia-radar-detector-ban
Ok, someone explain this to me. I put my my manual transmission into neutral at the top of the hill, and coast. According to the linked PM article, I’m not saving gas, even though the tachometer indicates about 700-800 rpm (IIRC) while coasting.
If I kept the car in 5th gear on the same hill, at the same speed, the engine would be running at 2000 rpm, BUT it wouldn’t be using more fuel? What, just because I’m going downhill the cylinders aren’t firing, and not consuming gas? What’s going on inside the engine cylinders while I’m in gear going down a hill? Not to mention I have to keep my foot on the accelerator, or I’ll engine-brake, slow down, and eventually stall.